Another thing they do is speed up and simplify doing the dreaded "Tactical Reload".
Obviously with a handgun, you grab a magazine out of your pouch, move your hand up under the mag well and dump the old magazine. The general idea is to hit your palm is such a way that you can pivot the partial magazine between two fingers where it is held and you insert the full magazine. Obviously a rifle magazine is considerably bigger than a handgun magazine and doing a true tactical reload is difficult without dropping anything. With a Mag-Pul, you stick your little finger through the loop and punch out the old mag.
PLEASE let's not turn this into a discussion about the pros and cons of doing a tactical reload, just know that if you intend to do one, a mag pul will make it easier. If you don't, it doesn't matter.