The purpose of a foam gun is to reduce wash induced marring and scratching by keeping the surface flooded with slippery suds that allow your wash mitt to glide over the surface and help suspend dirt particles away from the finish. Of all the detailing products I have purchased over the years, my foam gun was by far the best investment and most fun to use. It actually makes washing your car fun much like using a squirt gun was fun when you were a kid.
Patrick