Referential transparency and higher trust ability when reading code(imo).

I think pure functional is a stricter but “purer”(no pun intended) way to reason about code anyway vs impure functional/purely imperative styles.

You can absolutely use these benefits in some other languages, at least from my (somewhat limited) experience. Racket (scheme) is one example, but another, perhaps more popular being Scala. (monads)

I’ve yet to dive deeper into Haskell tbh but you raised an interesting observation & who knows I might finally get into it haha