I'm sure you've seen that little bit of spray that ends up on your car's body right after the wheels? Mudguards prevent that mess. My g/f's Camry has mudguards and her car stays clean there, even if she hits a puddle. I constantly have to clean my car cuz I don't want that crap caking onto my paint.
Dealers sell the mudguards. Get it from Circle BMW or Pacific BMW for the cheapest price, but they're only like $20 per set (you need a front set and a back set) so after shipping, you might just wanna get it from your local dealer.