[lmm]

Essentially any library function that takes any kind of callback benefits from being able to take a generically-effectful callback. There's plenty of mileage in basic things like database row mappers.

If you want fancy examples, things like generic data structure traversals (I use a cataM like https://github.com/ekmett/recursion-schemes/issues/3 all the time). Another good example is iteratees - you can define an effectful source, an effectful sink, or an effectful transformation stage in a stream pipeline, which makes it very practical to have very small reusable pieces. Since it's using the generic monad interface you can define an intermediate stream transformer even for some custom effect that you wrote yourself, but be confident that everything will be plumbed together correctly.