Most game designers, especially the ones in the industry, know that the more hours you get out of your game the better it will sell. Games like Skyrim are made to pull players in for hours and hours without the players getting bored of the game.

Of course, not all "long" games are worth playing, and some companies are very good at pulling players in for hours and hours while failing to deliver an overall satisfactory experience that players could get from other games(Like Call of Duty). But these games can still sell very well, and that is why it is important for all game designers to understand repetition and how to make players do the same things over and over again without getting bored.

It is almost always guaranteed that players are going to be doing the same thing over and over if they are playing a game, especially for a long period of time. It's the job of the game designers not only to make this fun but also to make it so the players don't get bored of it. The magic of excellent modern game design draws players in to the world, creating an illusory sense of mystery and forcing the player to always be intrigued by what they may see next.

There are many ways to make your repetition work. One way is to consistently introduce new game mechanics encouraging new game-play styles up until the very end of the game. For example, in Batman: Arkham City the player will be seeing new enemy types and new game-play challenges right up until the final level. The game never stops throwing new elements of game-play at you, and the player almost feels as if they are always learning something new about the game. For example, on the very last level the player is introduced to stun baton wielding enemies that must be defeated a certain way which only appear on the main quest 1-2 times. On top of that, the Riddler Challenges constantly encourage the player to try new and unique things. I could go on for a while about Arkham City's excellent use of repetition, but I'll save you the tl;dr

You don't have to always be introducing new game mechanics to make your repetition work well. In fact, games like Skyrim can make you do the same thing over and over again without it ever getting boring. This is because of the illusory sense of intrigue that well designed worlds have. As you adventure throughout the land of Skyrim, you will encounter hundreds of different dungeons. Almost every single dungeon has its own unique story and this constant refreshing sense of being able to experience something totally unique in every level allows players to be able to do the same things over and over again without getting bored. The game as a whole has a sense of world, as if it is alive. Players will spend hours and hours playing the game even if that means doing the same things again and again because the game presents these tasks in a unique way every time.

I hope that was informative. There are plenty of other ways repetition is used and repetition is -far- from being the only element of game design that i could go into discussing but its a good one nonetheless and its on my mind
I believe the term you are looking for in the last 2 paragraphs is "Immersion" rather than repetition. Which is equally important in my opinion. If players can immerse themselves in the world you created and relate to their character in a sense its almost guaranteed they'd be playing the game a lot longer as opposed someone who couldn't immerse themselves in your world.

Immersion and repetition are two very, very different things.

Immersion has to do with the atmosphere.

Repetition has to do with the game-play. Repetition is getting the player to do the same thing repeatedly without them becoming bored of it.
Immersion also ties in with game play as it relates to certain mechanics working together and not just atmosphere alone.

I was mainly pointing out this line:
games like Skyrim can make you do the same thing over and over again without it ever getting boring. This is because of the illusory sense of intrigue that well designed worlds have.

With Skyrim Players can do pretty much whatever they want which contributes to immersion factor. Doesn't that tie in with game play as well?
Skyrim has many features which enhance the atmosphere in a brilliant way, however this still has nothing to do with the way that the game makes players play through the same/similar dungeon setting over and over again while still enjoying it every time. Sure, you could say that the repetition is enhanced by atmosphere but in no way is repetition the same thing as immersion
I said, I thought you had mistaken the two. I said Immersion tied in with game play. I haven't actually played the game so I could be mistaken here:

I'm sure the game doesn't have players play through the same dungeon/content over and over[In the exact same way] again as part of the main storyline.but its the players who choose to do so because they are so immersed in the world the developers created. The fact that players have the option to shape their influence in that imaginary world increases immersion. Only reason i could think of for playing through the same dungeon/content repeatedly is to reap some sort of reward.

If I'm not mistaken it was Skyrim's dynamically changing world, plethora of content,game play, and that overall sense of freedom that kept players hooked for hours/days on end.

Skyrim was one of the most bland and boring games I've played in recent memory. The most suitable expression I've heard to describe it would be: Skyrim is a vast ocean which is only a foot deep. There's tons of stuff in the game, but most of it is only superficial. Clubs behave the same as swords, as you level up you're locked into a certain play style, the NPC's just stand around waiting to be talked to.

What a good game needs is depth. It's the difference between Dark Souls and Skyrim. Depth is the reason why I would spend hours getting all the dumb achievements in Dark Souls while I couldn't even bother to finish the main quest in Skyrim.

Dark Souls focuses on doing fewer things extremely well. About 70% of the game is walking around with your shield up while trying to get a hit in on an enemy. You do this from almost start to finish yet it never gets boring. This is because the combat is so rich and varied; most weapons behave differently, enemy AI takes full advantage of their abilities, the leveling system prevents you from being locked into a certain play style. Besides all that, the friendly NPC's are all on their own quests which don't revolve around you. They move around the world as you progress through the game, what you do effects the outcome of their individual journeys.

Repetition isn't inherently a bad thing, but if handled poorly it can ruin a game. Repetition is actually what makes a lot of casual games so successful.
Yut Put wrote:
There are many ways to make your repetition work. One way is to consistently introduce new game mechanics

You realize you're saying:

There are many ways to make your repetition work. One way is to not make the game repetitive.

Right? =)

Repetition, in some form, has to be there. Imagine a game without repetition. You'd press one key one time to use one weapon against one enemy and that'd be all of the combat. Anything more would be repetitive. Nobody wants a game like that. The key, which is what you seem to be getting at, is that you can't have everything be repetitive. Whether it's having unique dungeons, introducing new enemies, or rewarding repetitive actions (new abilities, medals, etc.), there has to be something that keeps the game from being boring.
The best example of something like this I can think of is actually a fairly old game. Diablo II: Lord of Destruction. The game has you play through the same five acts, story, everything countless times: yet you play it for hours in single player and days or longer in multi-player. Well, rather people did back before it became a bit too old.

Granted, it is a rogue-like, so one could argue things change often; however that's not really what does it. It's the same thing mentioned above. Small, but essential changes. You will often move through areas more than once to get the equipment or levels you need to move on, but its not a grind its a choice for easier fights. Some bosses are great for finding rare equipment if you kill them repeatedly. Beating the game results in a new mode where everything is more difficult simply because the monsters have new names, stats, and a few more abilities; plus beat it again and the same thing occurs once more.

Repetition can be the best or worst thing in the world for a game. It's all about how creative you are in the way you use it, and how well you mask the repetition. I'm echoing Forum_Accounts points a bit here, unintentionally, but that game is a really good example of them and then some, in my opinion.
There are some pretty good papers on this. Several of them analyze it it terms of platformers. Generally a successful game will mix training (that is acclimating players to play elements) with new experiences and mastery tests. Take jumping as an example. The early game may train with simple jumps with minimal consequences. Then, it may add jumps with damaging obstacles/enemies/etc. For later stages it may add difficult jumps that result in death on failure. All of them use the same mechanic- pressing the jump button, but by varying timing, consequences, etc. it keeps it from getting stale. Likewise, once the player is comfortable with a mechanic, you can add variations- in this case, spring blocks, double jumps, etc.

If a player feels no reward from an activity, it will quickly lead to boredom. This is why games often throw in variations. But it actually has to present a new challenge (and not just be frustrating) to work. This is why QTEs usually suck- they are different, but they are usually either frustrating or mind numbingly easy. Even worse is long, unskippable cinematics as they are completely uninteractive.
Your game can be very repetitive as long as it's super-addictive. They might not be good games, but they still get a large number of players because they become addicted. Ex. Call of Duty, anime games on here, Farmville, Tetris even. Then just add periodic content, or new version releases and you're set, because they're still addicted but you've added something new so that they aren't tired of playing it.