I also like to tint primers to get a better color. If you are using the
latex white primers (Kilz comes to mind), the paint store will tint it
for you. This helps a lot.

Also, with paint colors like red enamel, a prime of white makes the
color brighter, as some colors are somewhat "transparent," and do not
cover well.

Also, primers do wonders to level out the minor imperfections in a
surface, as well as giving better adhesion for the final coats.