This is a tool that we do not use too often, but we do use it on a car that we have never washed before and going to wax or on a very dirty car. We also use it on a heavily bug plastered car too. The main benefit is that you can pre-soak your vehicle with shampoo without the need to touch your paint. This allows the surfactants in the shampoo to start lifting contamination from the surface of your vehicle, which can also speed up the wash process.
Here is a video on foaming a car