A primer sealer, in particular a wood primer, is most essential when painting surfaces as it penetrates the surface in question and allows the products that are applied on top of it to adhere well to the surface.
This is a piece of wood timber that has already been primed a nice even coating ready to receive a good undercoat on it.
An undercoat fills imperfections on a surface, be it grain marks or ridges.
A surface that has been applied with an undercoat will be flat and most ready to receive the finishing coat especially if you are using paint with a gloss finish.
It is certainly more economical to apply a layer of undercoat than to apply more layers of a more premium finishing coat.