Class types as "convenience functions"

I had hard time understanding the common practice in haskell of MonadXXX class relationship with XXX type (e.g. MonadReader and Reader) until I run into this line:

The MonadWriter class provides a number of convenience functions for working with Writer monads. In the Writer Monad tutorial.

Looking at the class interfaces as "convenience functions" to work with underlying type is interesting way of looking at it.