I have a thesis about one of the reasons that earned experience points are so persistent in roleplaying games.
Experience points provide an irreducible floor of plot.
As is traditional in RPG theory, I’m using an idiosyncratic and non-intuitive definition of “plot”. What I mean by “plot”, in this connection, is the occurrence of events which cause a lasting change of state in the characters or setting. Put more simply, plot is when something happens that matters.
Ideally, in every session of an RPG, there would be plot. PCs would advance toward short-term or long-term goals, gather information or assets necessary to advancing toward goals, move along a character arc, etc. However, we’ve all had sessions where nothing happened. Maybe there was some beating on orcs, but nothing really happened. Nothing changed, in the characters or the world they live in.
Experience systems, however, guarantee that the PCs will be changed by every session, regardless of that session’s content. Whatever else happens (or doesn’t), you get XP, and now maybe you can get that new ability you’ve been saving up for. They turn otherwise narratively vacant sessions into training montages.