Because immutability is a massive boon to horizontal scaling and maintainability in ways that are simply unattainable in languages where it's not encouraged. It makes concurrency and parallelism observably trivial, it makes your code safer. Assuming you're using a strictly evaluated (AKA eager, greedy, whatever else you know to mean not-lazy) language with immutable data, race conditions are eliminated.

Let's review that again. In a large distributed codebase written in an immutable language, (let's say Elixir, it's fashionable at the moment and good for doing this kind of stuff) concurrency and parallelism are trivial and race conditions are nonexistent. This is even before we get to type safety and stuff like the actor model. That's huge. That's the immutability advantage. I could go on but it's late and I have school tomorrow.