[Haskell-cafe] What is MonadPlus good for?

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