The whole point of Monad is composability. In the category in green, T -> Monad<U> and U -> Monad<R> are not composable. With the help of fmap and join, we map them in to the category in blue (actually the category itself), they become composable.
Monad explained in one picture
原文：http://www.cnblogs.com/weidagang2046/p/monad-explained-in-one-picture.html