Some people will tell you to use paint as a primer or use primer as a finish coat.  They are not meant to be interchangeable; paint contains pigments and primers contain resins.  What does this mean?  The resins in primers serve specific purposes, there are various primers available for different scenarios, sealing new drywall, sealing bare wood, blocking stains etc.  When primers are used correctly they provide a stable surface for your topcoat.  What about “Paint and Primer in One”?  There is no actual primer in the mix, it is however thicker and builds higher giving you a sturdier coat of paint which is great for painting over previously painted surfaces.  Primer is not required in every situation but your painting contractor should know when it is needed to achieve the best finished product.