If you brake with your legs, then you save money on brake pads. Rim brakes don't work so well in snow or slush.

I think in most cases its just cause you want to. I usually prefer the freewheel. When I ride fixed, its just for fun. Last time I road fixed, it was on the bike trail with my wife. I don't brake exclusively with my legs, but it was fun checking my speed without grabbing the brakes.