immutability

Is there a benefit to immutable data structures like in Clojure and erlang? What does it do for you versus just not mutating mutable objects?

>>1
Immutable data means you don't need to lock concurrent access to variables since a change will never be observed. Clojure seems to be more sophisticated, IIRC >>2 doesn't apply as much to Erlang as it does to Clojure.