well in really fast sport cars, and f1 cars, the spoiler helps to force the car down on the road.. its like an inverted airplane wing, the airplane wing helps to lift the plane, the spoiler pushes the car down on the road providing more grip, and if it wasnt there, we'd be seeing merc clk gtr's, mclaren f1's, and all the f1 cars taking of like planes..
in paki cars , hondas and corollas the spoiler is just for show, mak the car look reely reely sexshy....