Primer coatings are designed to provide the surface for the finish coats of paint finishes.  Primers seal the surface off and provide adhesion for the finish paint, they are used on bare metal or previously treated surfaces.  Primers insure longer lasting paint work as the resins in the finish paints stay on the surface creating the "wear layer" as they are designed to do.  Peeling and premature failure is eliminated and this is by far the most important part in getting a long lasting, durable finish.