Unless they're on performance cars like a Mustang GT-350 or Shelby GT-500, most of them are pretty much cosmetic things meant to make cars look cool. Nobody drives fast enough on the street for a chin spoiler or a deck spoiler/ louvers to do much of anything aerodynamic. If they were meant to work well at highway speed, they'd have much more exaggerated curves to compensate for much slower speeds and create the stabilizing forces that they do on Indy 500 cars or NASCAR stock cars. Those spoilers are thin and don't have extremely exaggerated curves, because they're going 200+ mph. A chin spoiler on an EcoBoost Mustang is also thin and really doesn't do anything.

They are strictly cosmetic on street vehicles. The right one, on the right car, can look fantastic. But the trade off is usually scraping when driving thru dips, etc. 

cosmetic thing doesn't change anything unless the aerodynamics of it are terrible