Snow foam is designed to keep the moisture on the surface for a prolonged period in order for it to soften up and loosen all the surface grime, but much of it will come off with a pressure washer rinse before hand and the car being wet doesn't impede the effectiveness of the snow foam in my experience.
I tend to go around all the worst areas of the car (lower half of doors downwards, sills, wheels & arches, rear end, front bumper) with a 6:1 dilution of Valet Pro Citrus pre-wash applied via a pump sprayer and leave this to soak for 5 minutes before pressure washing off. I find that this shifts the worst of the surface dirt, bugs, and general grime. It's also pretty effective at cleaning the worst of the brake dust off. Once that's rinsed I'll snow foam the whole car before rinsing a wheel at a time and cleaning all the wheels properly while the snow foam dwells.
I find it takes me about 30-40 minutes to thoroughly clean all four wheels, which is perfect for the snow foam dwell time. Next step is to rinse the foam all off and this actually leaves the car looking superficially clean, though on close inspection it's still got a film of dirt on it. This film of dirt, however, is easily cleaned with a two bucket contact wash.