The historical context helps here. Monads came to Haskell to deal with IO. Once there, people realised that a whole host of things are monads under the hood, most of which are pure. The word "effectful", somewhat confusingly, is often used for applicative/monadic code that is in fact pure under the hood. A good example is the State monad which does not mutate state in any way, it's completely pure computation that happens to model state.