The big box stores employees at the paint counter seem as ignorant as a novice about the vagaries of application most likely because they themselves are novices. They may know quite a bit about colors and matching and how to work the mixing machines, but they probably don't have much more experience painting than the average homeowner. That knowledge comes more from doing, whether its a handful of projects as a homeowner, or regularly as a professional.

IMO, the stores could do better about offering basic information about this, about when one would need a primer, or just a topcoat, what kinds of paint bases are appropriate for what applications, what kinds of application methods are appropriate, etc. Instead, the response of the big box stores seems to be to skip the question entirely with combined products, e.g. "paint and primer in one".