These are especially prevalent on high-performance sports cars, supercars and most racing cars when putting spoilers that create similar downforce would just cause too much drag.
The double-deck diffusers used by a few teams caused a scandal in Formula One over the 2009 season.
See Simple Plan.
Oh My God My Girlfriend Left Me
There's nothing left in the world
I hope you're happy now
*A piss poor guitar solo*