Obviously, the faster one travels, the great the down pressure on the rear spoiler therefore pushing the arse end of the car down onto the road. This will give you more grip.
You may or may not have noticed that Porsche rear spoilers (in some cars) only appear when the car is travelling at higher speeds, this is to push the rear end onto the road, giving better grip, therefore better handling.
However, too much downforce will restrict the cars straight line speed. As such, a balance needs to be found between the two i.e. better grip and handling or straight line speed!
Not sure if this will help.
http://arjournals.annualreviews.org/doi ... Code=fluid