Immutability is an excellent tool to manage mutability. It lets you make guarantees about what won't change, which allows for some really great optimisations - like safe cross-thread reads without locks.

I agree it's not right for everything, but there are a lot of operations that can be modelled with immutable data, such as most server-side web code - reads definitely, and a lot of the write operations don't actually need to mutate objects though most ORMs and other DB APIs do require this.