Pro Changes to state are made with pure functions
Reducers are pure functions that returns the next state based on the previous state and an action (describing what should be changed). Since they are pure functions, they return a new state object instead of modifying the existing state. This ensures that it doesn't affect anything outside of the function's scope, making it free from unintended side effects and also makes debugging easier.
Pro State is predictable
In Redux, the state of your application is held in a single object. The only way to change the state is to emit an action describing the change — there is no way to mutate state from the view, callbacks, or anywhere else. This makes the state of your application predictable and objective, and reduces the chances of any unintended state mutation by a callback somewhere else in your code.