I wont say it improves performance, say with a normal street car that some bloak thought would be smart to drop... that only decreases the driving stability..

But.

On a race car built to go fast, again it doesn't improve performance but it does make the car suck down on the ground increasing stability...

But most people just drop them because they like the look, although I'm a fan of it but not with cars that drive on public roads...thats just common as hell xD