I didn't learn CT in school, but I got comfortable with the functional programming version of it during a few years of writing Haskell-flavored Scala on a team.

I didn't learn CT in school, but I got comfortable with the functional programming version of it during a few years of writing Haskell-flavored Scala on a team. To me, the main value in learning FP CT is becoming handy at recognizing more of the patterns underlying the code I've been writing and working on for ages. Consider the GoF Design Patterns book -- if you haven't been programming very long, many of the patterns might look odd or needless, but the more code you work on, the more you start to recognize the patterns as applied in real life. Category theory for programming is like that, plus with some added formalism that proves how it's safe/possible to compose these patterns.