You'll want to pre-rinse a dirty car to help blast away as much crud / salt / mud / pollen / grime as possible. It's a simple step to help reduce the possibility of introducing scratches during the wash process. The foam will encapsulate and emulsify road grime and surface dirt either way, wet or dry. I always pre-rinse, foam, rinse, and then either foam again and hand wash, or just hand wash depending on how dirty the car was to begin with.