Tigerdude,
Primer actually accomplishes several purposes. First it fills in any small scratches or imperfections in the surface. This will give a better finish in the final product. Primer also gives the top coat of the paint a better "grip" to the surface being painted. As in the other thread, I'm not a chemist so I can't explain the entire process in detail but I was taught that the paint needs to be considered an entire "system". Just like an airplane being a system, you have to have all the parts for it to all work correctly. You wouldn't try to fly an airplane without a vertical stabilizer, in the same way you shouldn't paint without the primer.

This may not be the correct answer, but that is what I was taught and that's the way I've always done it.

Ken