David, professional painters don't keep opened paint around very long—in the can it's inventory, on the walls it's income. For short term storage this is a "quick and dirty" technique instead of wiping the seal area completely clean.
But paint is a lousy sealant when exposed to liquid paint because it never quite cures completely, being constantly exposed to the solvent in the can or bottle.
If you keep the sealing surfaces of the jar and cap scrupulously clean, they seal surprisingly well. Paint dries out in the container when that seal is compromised by paint in the seal, wet or dry. If dry, it creates an irregularity in two otherwise mating surfaces. If wet, the paint dries, shrinks, and cracks, negating the seal.
If the cap liner is damaged, cut a replacement from thin cardboard (like on the back of a note pad) and cut a circle of aluminum foil slightly larger than the inside diameter. The cardboard deforms against the jar edge, pressing the foil tightly against the glass.