The way I see it is, just as in Object Oriented design patterns, category theory are the design patterns of functional programming. It just so happens that the mathematicians got there first and gave everything greek names.
http://logicaltypes.blogspot.com/2015/08/pure-functional-pro...
Not that being maintainable isn't a worthy goal or that expressiveness and DSLs can't be maintainable, just wondering why it would be the primary goal.
IS CT only used when writing a piece of code that's supposed to be long living and have regular updates or something?
Let's take something like Clojure. I get immutable data, sane multithreading (I haven't had thread related problems in my code for years), etc. All without monads. Whenever I've started doing things like introducing state monads, or seen code involving applicative and "free" stuff, it basically becomes an opaque blob of functions. Zero ability to have the code self introspect or to leverage that pile of functions.
The better method is immutable data and data-first designs. Data is searchable, introspect-able, and transformable. Most CT stuff is all about composing functions via functions. Making your entire system opaque, at least from the point of view of the program itself.