If you live in a place where roads are salted, washing your car is generally recommended to reduce rusting and paint damage. I don't go to the car wash often and I only use the one at my local gas station, but it's rare that I go there and there aren't already at least two cars in line. And I live in a very low population density area.

At 1 car every 14 minutes on average, a single bay will easily clear $1,000/day of revenue.