The main reason painters should always invest in a primer when painting a new wall lies in the drywall’s porosity. Bare drywall is extremely absorbent due to it being primarily composed of gypsum and paper. If you choose to skip the primer and apply paint directly to the wall, the applied paint may soak into the wall at different rates, resulting in a splotchy and uneven appearance even after applying the last coat. You likely won’t achieve the final look you want for your walls by avoiding a primer, even after several applications of paint.
There are some instances where primer isn’t needed for a DIY paint job to look uniform and clean. For example, you may be able to skip priming your walls if they’ve already been painted before, have been checked and treated for imperfections like cracks and dents fairly recently, or if you’re using a two-in-one paint and primer formula designed to eliminate the priming step for painters entirely. These self-priming paints aren’t ideal for all paint projects, as homeowners may find they don’t offer as even a coating as walls that have been fully primed beforehand. Professionals recommend using 2-in-1 paint only on already-painted drywall.