Primer is often used to get just get a good look at the piece. Often times you'll bondo, sand, fill, sand, fill, bondo, sand, and fill to the point that you can't get a good overall look at your progress because you're distracted by all the pink and red colors. So you prime it so you can see it all in one neutral color. It also helps to prime it when final smoothing and filling in pinholes because often times all you need to do is prime and sand.
