What are the advantages of the stateful approach in mobx?

Question

React

Hello. Please tell me about immutability. As far as I know from the pros, it's safer to use and test. And the approach, for example in Redux, is not to mutate the data, but to return its new state. Now the question itself, as far as I understand in mobx, on the contrary, the state is mutable, unlike Redux. Why, if mutating data is often, if generalized, bad, then in the well-known mobx library, the approach is to mutate data?