Finish paint is desgined to wear, washability, UV protections, ability to hide and similar features.

There are many different primers for different uses.

One is for bonding between "difficult" surfaces and the finish.

Another is to seal different in porasity of different materials. This type is commonly used one new drywall or else the difference between the drywall paper and the compound (mud) will show through.

Another purpose is to seal in stains that might bleed through the finish coat.

Different primers might only have one of those features or maybe several of them.