Redux is a strict framework. Whether you view that as a pro or a con, handling async actions can sometimes be a pain. Especially if you are using middleware such as Redux Thunk and have to chain promises.
As an application grows, it's difficult to maintain and continuously test these actions. By the use of the ES6 generator function, Redux Sagas break up async actions into synchronous events. This makes it easier to test, chain actions that depend on the result of each other, and deserialize promises.