All helps soften and dislodge dirt before you start dragging it accross your paintwork and putting in swirls and scratches, so a good step in some cleaning cycles. Usage does depend on what waxes and sealants you have on the car as with my car which has a long term seal (GTechniq from memory), snowfoam is no used as it is quite agressive and would also strip the sealant, and dirt doesn't bond as well to my car anyway due to the sealant, so a simple rinse before the wash is enough.
Hope that helps!