"Primers" are materials that promote adhesion. For painting automobiles, the primer promotes adhesion of the color. Without the primer, the color would quickly fall off.
Many materials have multiple characteristics. A "primer-surfacer" promotes adhesion, and also has enough solid content to allow sanding of the film to perfect the surface prior to painting color. A good quality 2K primer surfacer can be topcoated without any further application of sealers. Others are primer, primer-sealer, or sealer....
"Epoxy primers" have excellent adhesion characteristics, excellent corrosion resistant characteristics, and great durability. They work well as a barrier coat between old subcoats and new applications. They have good filling characteristics, and can be sanded and used as a surfacer, but I've never liked sanding them.
