Pre-wash foam as opposed to shampoo or a washing detergent for this stage of the car cleaning process. First of all, this substance is designed for softening the dirt on the car, which is not something regular detergent is designed to do. Additionally, while a car washing shampoo will often leave behind marks when it dries, a pre-wash foam will not cause such inconveniences.
By waxing your car, you're adding a protective barrier over the paint, since the clear coat of wax will take on any damage so your paint doesn't have to. So if you're looking for a way to protect your car from scratches and chips in the paint, waxing it is the way to go.