Purity is also not the same as immutability. Clojure has immutable data and impure functions, and it also offers good, safe concurrency features. Purity helps with other things as the parent comment mentioned, but it's not necessarily tied to immutability. But I maintain that purity is necessary in the presence of laziness, because it's terribly confusing to reason about side effects when you're building up thunks everywhere.

Can you sprinkle parallelism annotations and have guarantees about semantics not changing while gaining parallel execution?

Also, note concurrency and parallelism are two of many interesting benefits of purity.

I'll also add unit tests and property testing which are much nicer with purity.