Immutability

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

Not counting concurrency?
You can share the same copy of a value among all uses of the value. This extends to parts of the value (substrings, subtrees, or tails of lists) and across multiple functions and multiple threads. You have a guarantee from the compiler that the value will always be the same. If that value's a huge array (say, an image file) or binary tree, that's a big savings in memory and time spent making unnecessary copies.