a) know why some things are done the way they are (in case you're not satisfied with "this works!"), or
b) if you want to find inspiration to write a certain kind of library: Haskell is one of the few languages that has, as I like to call it, "over 50 years of documentation".
To elaborate: a category is a bunch of objects, and functions (morphisms) between the objects. The types in a language like Haskell form a category (called Hask in this case) where the objects are the types and morphisms from one type to another are the functions that go from the former to the latter.
A lot of things done in Haskell are instances of general constructions from category theory applied to Hask. Comfort with these general constructions certainly helps to illuminate many ideas.
For example, the great trifecta of Haskell abstractions comprises three interfaces ("typeclasses") called Functor, Applicative, and Monad, which are used everywhere. Each of these corresponds to a concept from category theory: for instance, what a Haskeller calls a "Functor" is what is categorically called an "endofunctor on Hask". Among users of the language who know any category theory at all, most have some familiarity with things at the level of what I've just said, but not much more. A knowledge of category theory at any higher level (like in the linked repo) is not at all common, statistically.
Also, not many of the common libraries are based on deep categorical ideas (lens  is an important exception) but even then, knowledge of the mathematics is useful if you want to dig into how the ideas are implemented, not if you're happy just using them. Learn as little or as much as you'd like.