Pure, fun math does benefit society directly in at least one way, however, in that the opportunity to engage in it can be used to lure very smart people into otherwise unpalatable teaching jobs.

Is it clear that this is in the best interests of society? It would seem to me the end result is bad teaching. Back when I was in undergrad, the best researchers were the worst teachers (for obvious reasons- they were focused on their research and didn't at all care about teaching).

When I was in grad school in physics, the professor widely considered the strongest teacher was denied tenure (cited AGAINST him in the decision was that he had written a widely used textbook),etc.

Also, the desire for tenured track profs to dodge teaching is why the majority of math classes at many research institutions were taught by grad students.