This `do` syntax desugars to binds like:

    ma >>= \a -> mb >>= \b -> return (a > b)

We can then inline the definition of `>>=` and `return` to get:

    case ma of
      Nothing -> Nothing
      Just a ->
        case mb of
          Nothing -> Nothing
          Just b -> Just (a > b)

Imagine that `mb` is actually a really expensive computation that we don't want to perform unless `ma` returns a value. Sequencing our case statements in this way allows us to do that. `mb` will remain an unevaluated thunk until `ma` evaluates to a `Just a` value.