The idea of snow foam is to take dirt particles off the car so that you don't rub them into the paint with your mitt when washing. So if the cars just dusty then although there's not much point it does still help a bit, but its far more effective when the car is dirtier obviously.
When the car is only dusty some people just snow foam/ rinse and then dry the car so you don't have to touch the paint with a mitt but personally I never do this as its never properly clean.