To a much lesser extent, due to the angle/height of the wing. I'm NOT saying that anything taller than the ITR wing would be better. But it is a fact that the higher ITR wing (and different angle of attack) is more effective than the GSR wing, which is much more cosmetic.

It creates downforce, which keeps the rear end planted firmly. It is especially helpful when trailbraking at high speeds (above 80mph or so). It is also helpful at high speeds in a straight line, but not as much as while trailbraking.