[Haskell-cafe] What is MonadPlus good for?

[Benjamin Pierce]

I have seen lots of examples that show how it's useful to make some type
constructor into an instance of Monad.

Where can I find examples showing why it's good to take the trouble to show
that something is also a MonadPlus?  (I know there are many examples of
things that *are* MonadPluses; what I want to know is why this is
interesting. :-)

Thanks,

     - Benjamin