Well, the nice thing about this abstraction in Haskell, is that you can give a name to the notion of receiving a bunch of values (Reader), outputting a bunch of

Well, the nice thing about this abstraction in Haskell, is that you can give a name to the notion of receiving a bunch of values (Reader), outputting a bunch of values (Writer), threading state that can be both Read and Written (State), non-determinism (List), etc. You can thus compose all these behaviors you want into a single type constructor and build your programs as composites of this type.