Why use primer?
Primer is formulated to work with the topcoat of paint to promote better adherence to the wall. This is important if you are changing the sheen from a slick semi-gloss to flat, or wanting to cover oil-based paint with latex. New surfaces like bare wood or drywall require primer, which sinks down into porous areas better than finish paint. Primer is also needed to cover rich color?red is infamous for "bleeding" through?and taking the extra step has been traditionally well worth the time, in addition to a total savings in the end (primer is usually about half the cost of paint, gallon for gallon).
What about all-in-one paint and primer?
Does it really cover in one coat? Will it do the job of surface adherence, or block stubborn mildew or smoke stains? Yes and no, depending on the situation. The reality is that two coats always are preferable to one. Self-priming paints are not new, and most state you will need enamel primer to cover watermarks or other stains.
Try the all-in-one paint if the sheen and color is close from old to new coats?but keep in mind that any paint would cover well in that situation. For problematic situations, use primer and paint in a brand you trust (they'll be formulated to work together), and have the primer tinted to the topcoat color. By E. E. Kane