It's kind of tautological, but the main advantage of immutable objects is that they can't change. When objects can change, you have to think about what might happen with them. You have to think about how, when and why you want to change them. You have to think about what other code in your application might have access to the same object, and what it might change without you knowing. Immutable objects effectively reduce the number (and granularity) of "moving parts" you have to juggle in your system and make your life easier.
