Primer is formulated to bond tightly to the existing surface and create a uniform, stain-blocking base, rather than to look good on its own. Painting a light color straight onto a dark wall usually means the old color bleeds through faintly no matter how many topcoats you apply, since regular paint isn't designed for heavy opacity. A primer (especially a tinted "gray" or stain-blocking one) neutralizes the old color in one or two coats, so the actual topcoat color only has to do the job it's designed for.