Typeclass systems should ideally allow users to define typeclasses that are supertypes of existing typeclasses. For example Selective is a superclass of Monad because it admits an implementation selectM that is defined for all Monads. A library providing Selective however, can’t anticipate every possible type the user might use. So it would be useful to be able to insert a default implementation of Selective for any Monad. e.g.

-- this does not compile in Haskell as is because resolving this type of
-- constraint is hard
instance {-# OVERLAPPABLE #-} Monad m => Selective m where
  select = selectM

Haskell has needed to revise its typeclass hierarchy because a new better formulation came along that required reparenting various type classes. The primary examples of this are Semigroup => Monoid and Applicative => Monad. These revisions could have been made easier by providing a default instance potentially.