Primers are not always necessary in every painting project.  To know when you should use a primer you first have to understand what types of primers are available and what they're intended to be used for.  The best way to understand their intended uses is to first look at them by primer categories. New/bare drywall is extremely porous and soaks up paint like a sponge and causes it to cover better in some areas than others; especially when you’re comparing drywall mud joints to the surrounding areas. Porous surfaces like drywall mud will cause visible differences in the final sheen of your paint called “flashing”.