[dddbbb]

I'd say there is quite an important conceptual difference in that the state monad example exactly desugars to pure functional code. Monads give us a good syntax for sequential computation, but this sequentiality is built out of purely functional components.

[lalaithion]

The State monad desugars to pure code, but the IO monad doesn't. If you use IORefs or MVars, then you do actual, real mutability by changing the value at a location.