If you're painting unpainted brick, an acid wash is usually called for. Problems also can be hidden inside interior surfaces, too. For example, recently applied plaster may be a tough surface to coat. Plaster and masonry surfaces should age before painting. How long depends on the particular surface.
Acrylics figure in priming unpainted drywall, too.
Because an acrylic primer guarantees against raising the grain of the drywall paper, where regular latex doesn't. As some painters like to say, a textured surface wasn't in the bid. Sealers and primers help even out the porosity and give you a better-looking finish. A few common difficult surfaces have thus far gone unmentioned. There are enough coating problems associated with concrete to fill a year's worth of magazines.