Category Theory was called abstract nonsense by theoretical mathematicians, but it has helped improve type theory for computer language design.

G.H. Hardy was a British mathematician who expressed pride in the "uselessness" of his work, believing that pure mathematics was an art form that should not be tainted by practical applications. Ironically, his contributions to analytic number theory now underpins modern cryptography.

It’s weirdly difficult to study something for years - no matter how abstract — and successfully avoid any practical applications!