[acemarke]

Perf optimization is a big benefit, yes, but it also fits with functional programming principles in general.

React's `setState()` definitely doesn't care if you mutate or not - you can `.push()` right into an existing array in state, and re-set it into state to queue the re-render.

On the Redux side of things, immutability is important for several reasons. First, pure reducer functions are more easily testable. Second, they enable time-travel debugging - without immutability, jumping back and forth in state would cause the state contents to behave unpredictably, breaking time-travel. Third, the React-Redux `connect` function relies on immutability checks against the root of the state tree to see if it _thinks_ anything has changed, and against the return values of your `mapState` functions as well. If you mutate Redux state, your connected components usually won't re-render properly, because they think nothing has changed.