A clean car, a clean conscience
My father was hardly a car person and our family did not own a car until I was about ten years old. Then he relented and bought a used car from the local telephone company where it had been driven hard by salesmen.
There was no back seat, so my two younger sisters were provided tiny folding seats that faced each other. Finally, after two years in that cramped space, the family moved up in 1950 to a new Buick Special. And it was that shiny vehicle that gave rise to the weekly car washing event.
I guess the logic behind the car washing was that the investment in a new car made so little financial sense any reasonable owner would feel the need to care for the car so it would last