Immutability

Aside from the case of reads and writes of reference types in the context of concurrency, what's so good about immutability?

>>1
As >>2-san noted, concurrency is good enough a reason. However the direct consequence of immutability is that it enables referential transparency and memoization.

http://en.wikipedia.org/wiki/Referential_transparency_%28computer_science%29
http://en.wikipedia.org/wiki/Memoization