If there's a point to any of this, I think it's monads let you package up some algebraic structure in the form of a function `m a -> a` that satisfies certain properties, and then you can pass such a function around without any reference to the fact that `a` implements that algebraic structure. This function is the interface. (Is there real practical significance? I'm not sure, but I'm not trying to argue that :-) )

> all the interesting bits come from `a` being the monoid in that example.

Yeah, there's nothing deep there. I just said what I said to underscore that the converse is true, that if anyone were to hand you such a `list a -> a` function then it's encoding some monoid structure on `a` (one of often very many).