snowfoam is like a pre-wash soak. the foam lifts the surface dirt etc away and as the foam falls off the car it takes the crap with it, so when you come to washing the car there is less chance of there being harmfull crap on your paint. in all honesty, my old golf was very well cared for and waxed so in summer i have snowfoamed, rinsed and not much else.
i once tried snowfoaming, rinse, snowfoam then washed with the foam and bucket of water to rinse the mitt and it seemed to work. not sure if i'd do it with a very dirty car but the principle is the same as just wahsing i guess?
plus it looks cool and the neighbours are always impressed/shocked/in disbelief!