First, you spray the dry car with foam which loosens all the dirt and washes off the grit, then rinse the foam off with the pressure washer and finally, wash the car as normal.
If you don't foam the car first then you're basically washing the car with a gritty sponge and a bucket of dirty water. Pretty much like the £5 hand-job, you get from the Albanians in Tesco carpark
As I can't be arsed washing my cars more than twice a year I tend to foam, rinse and let them drip dry. In fact, the roof of my Range Rover has never been washed since new.
That's called a car wash.