That random thing you learned for fun will show up years later like a secret key.

Knowledge does not stay in the category you filed it under. An odd interest picked up with no purpose tends to resurface in a situation nobody could have predicted, because most useful thinking comes from connecting two unrelated things. Curiosity that seems wasteful is how you stock the shelves.

Follow the thing you find interesting even when it has no obvious application, and give it real attention rather than a glance. Learn it properly enough that it stays with you. The payoff arrives on a schedule you do not control, which is why it cannot be planned for.