What is the Purpose of a Paint Primer?

What is the use of a primer for painting? It's not just a matter of primer vs paint. They work together. The primer is the undercoating that is applied before the paint coating. Primers fill the pores of the surfaces to be painted and increase the adhesion of the paint. Common primers include oil based primer, latex primer, and shellac primer. Some volatile organic compounds are produced during the priming process, so adequate protection is essential.