I've hypothesised before that learning math might be useful because a) you get lots of practice in understanding abstraction and how abstract objects can meaningfully be manipulated using rules, and b) you hopefully learn that proofs are nobody's opinion. So basically a lot of practice in using basic logic. Neither of which require study of useful or existing things.

Though obviously it would be preferable if the actual content were about useful stuff as well, to get double the benefit, it's not inherently useless.