I use the Autobrite snow foam lance on a Nilfisk pressure washer and Autobrite magifoam.
The idea is the same as cleaning a bowl with dried porridge/weetabix on it – do you put it in the sink and immediately start scrubbing? No, you immerse it in some water for a while and then come back later and just wipe the softened food straight off.
The problem with a car is that you can’t immerse it and plain water just runs straight off the vertical surfaces. So… foam.
You still have to rinse the snow foam off (carrying with it the worst of the stuck on crap) and wash as normal.