A self-serve car wash, in my mind, is generally someplace that you take a car that is beyond dirty, the automatic car-wash people denied you, or you just want to spray someone with the high pressure washer.
With that said, the car wash does what it is supposed to do. Pump in some quarters and wash away!
I sprayed down the engine with my own goodies to help. Maybe this isn't allowed, but who's going to stop me?