Treated/spot free water is the number one reason to use the car wash. Rain water is evaporated water, the same as distilled in which only the water molecules evaporate leaving the minerals. Minerals can also be removed by water softeners which attract and collect the minerals in a tank of beads. The beads are periodically flushed in a salt brine solution to dissolve the minerals and then the brine is flushed into the sewer system. (Many SoCal municipalities have banned the use of water softening systems as the municipalities are responsible under the Clean Water Act for cleaning the brine before it can be expelled from the city system.) These same minerals cause the corrosion of your plumbing fixtures.
My city water comes from a well/aquifer that has holding lakes that replenish the water by trickling through the ground. Lots of calcium in the water.
The number two reason for using the car wash is it is better for the environment. Good for the plant and animal life. The local car washes by us, I believe but not positive, are required to filter and reuse much of the wash water. The waste water does go into the sewer system to be treated. We are ASKED to not wash cars in which the water can drain into the San Francisco Bay. (Driveways. Asked but not required.) Asked as well to control lawn irrigation runoff that contains lawn chemicals such as insecticides, herbicides, and fertilizers.
Vinegar dissolves the spots.