I think of React as a functional language, not exactly, but as a similar mental model. Once you start thinking of React in such a way, a lot of their decisions make sense. Just as in Haskell we need to use somewhat more convoluted ways to make things work, such as monads, we do the same in React, using useEffect to encapsulate side effects (hence the name). We also keep the tree immutable for much the same reason as in functional languages, because immutability tends towards fewer bugs as we don't mutate internal state, but we optimize such immutability through things like a virtual DOM.
