As kim said, they are designed to provide downforce, just like in formula 1. :-)

I believe they can provide some meaningful grip in the rear, especially if the angle is adjustable, some are...The CF sti ones are, but yeah you need to be going quite fast..round corners to benefit...which most people dont do...Maybe adds in high speed stability, even in a straight line....under heavy braking from say 160+?

__________________
K.E.R.S!!!1