Why learn something new? To increase your knowledge. If that's not reason enough then nothing else is.
To me, that variation is implicit within such a question.
Edit: I'm reasoning from the HN comment guidelines [0]: "Please respond to the strongest plausible interpretation of what someone says, not a weaker one that's easier to criticize. Assume good faith."
This guideline is in fact a restatement of one of the fundamental principles of reasoned discourse, the principle of charity [1]
I have to ask- like what? Are we talking about the next javascript framework, here, or about, I don't know, modal logics or category theory?
I know a guy who got an internship at ARM during a PhD in modal logic, so, far as I can tell, modal logic is both intellectually engaging and also practical, as in useful, in practice (to be blunt: to land you a job). Is that what you are talking about?
Generally though I think it's fair to interpret any question of "why should I bother learning X" not as a statement that nothing should be learned unless there's a direct benefit, but instead as the question "what are the practical applications of X". I think most people here would agree that learning for learning's sake can be fruitful & rewarding, so there's no reason to assume otherwise in response so such a question.
Maybe they prefer to learn through building something. Maybe they're working on something and are interested to know if Prolog could help them with their project.