I'll add to this that the "abstraction" component of this description is key and the use-case for a gen_server is far more broad than what you might expect in say a networking or web context.

You would use them for instance when you need to group behavioral units with a defined interface, even if you don't require any explicit networking.

This is a bit reductive and wrong in some ways but think of gen_server modules and instances as "sort of" like classes/objects. You "send messages/requests" instead of "calling methods," but the spirit is essentially the same -- it's a way of modeling the world with code