It's a magnet for dirt and wax buildup. The downforce value of it is pretty questionable. Still, it's yet another tiny visual clue that this is not just any old 5 series. At the time I bought mine and even now I don't have a strong opinionn about it, but I figure it's better to have it for free than get nothing. (Just like the front license plate bracket I'm not using. It's even painted to match! Sweet! Maybe someday I'll move somewhere that enforces front plates...)