If I'm right at the moment it is like this (simple version):
Server creates icon>Turns it>Saves into RSC>Sends to Client>Client saves to RSC>Displays

If it will be made like this:
Server sends angle to client>Client turns icon>Client saves to RSC>Displays

1st. It's little shorter, and will decrease server load
2nd. When using turn(Icon, Angle) server won't keep sending icons to Client over again and again
3rd. It will be useful for some games, example: Kunarks Real-Time Tile-Based Line Drawing Library, it saved each angle of line into icon, makingi t ~120kb size, with this little update icon will be ~1kb size, and server won't need to generate new icons everytime

Practical use:
In shooting games laser beam can fly not only straights lines (0,45,90,135,180,225,280,315,360 angles) but with any angle without extra load on server.

I think some developers will be happy to lower load and smaller RSC files
This would kind of be a feature request for client-side processing, which is something wanted for a long time and not limited to a specific function such as turn(), but will not be implemented [at least in awhile] and has been discussed (you can search for it for more info).
Although, since 4.0 the client already does quite a few icon processing functions such as dynamically resizing them, it's possible that they may actually add turning too in a future update (if so I'd request to extend this onto the ability to turn() overlays/underlays as well, if feasible). Note that even if this is implemented the server may still do the turning itself as well and store the new icon into the .rsc, but it won't need to send it to the clients, yes.
In response to Kaioken
The main reason why I requested it because it uses a lot bandwitch if server will rotate all missile/arrow icons, but thanks for reply