It's not just about immutability. it's about the fact that you can do mutations producing a cheap copy of the data structure. If I have an array of ten millio

It's not just about immutability. it's about the fact that you can do mutations producing a cheap copy of the data structure. If I have an array of ten million numbers. I can alter one of those array elements, getting back a new immutable array with the modification. And fast. Yet the original array still remains without the modification. How the implementation magic works is not a concern when thinking about the abstraction. But you can read how the magic works and like any magicians trick, you end up saying "oh, so that's how it's done". So you can modify one array element and get the performance (extremely close) to how you expect a single array element modification to perform. You the original copy of the entire array also remains if some other code has a copy of it. Imagine a recursive algorithm searching a game board with different move possibilities. Apply a move to a board and get a new game board. Yet the original game board remains unaltered. And you didn't have to implement any magic in your search algorithm. No copying of the entire game board. You just get your new game board with the new move applied so that your recursive function can proceed.