Benefits of immutability are same as benefits of forbidding of global variables: no side effects. If function foo(a,x) cannot have any side effects, then you ca

Benefits of immutability are same as benefits of forbidding of global variables: no side effects. If function foo(a,x) cannot have any side effects, then you can compose it freely. Monad shines with containers, because you can write tons of useful functions, even your own libraries, for a container, which will operate on generic type T, so you will have lot of functionality available out of box for any new type T, which can be wrapped into container, without any additional effort. It's as powerful as templates in C++, but WITHOUT SIDE EFFECTS, so you cannot shoot yourself in the foot.