it makes it easyer to see the difference between high and low spots. you apply it so you can see the difference with what part is sanded and what is not, parts that stay black are the lower spots. so you know where to sand more, and where not to sand.. or what part maybe needs a bit more putty to fill up. you can use it when sanding with 80 grit on putty as a guide coat, or use it when sanding with 1000+ sandpaper to see if all the scratches are gone before painting.
