I am hard pressed to find any practical use for Category theory. No that is not quite true. Knowing concepts like the fusion law, natural polymorphisms, initial and final algebras, ana/cata/hylo morphisms make many fuzzy patterns concrete. And motivate many algorithms. But I have never found a use where a theorem was useful in deriving some part of a program. I feel it helps my programming the way some people say Lisp helps their Java. But I have flipped through Bird's Algebra of Programming a very few times to solve some problems.

the above is paraphrasing a recent post.