3 Answers

Primer is supposed to be of a different viscosity, and have a better saturation and adhesion for later coats to adhere to. I use it when it is recommended, especially for exterior applications. I honestly don't know if it helps, or if I just got good results from a good paint. But, I have had exterior paint stay looking good for over 10 years using primer...and that is not too bad. Good preparation and a clean surface make a big difference also.

You don't have to prime the walls. If you don't prime, it just takes more coats of paint. So, prime the walls, you limit how many coats may be required to complete the job.

If the wall is non absorbant it helps the paint to adhere, otherwise it may flake off.