Because a data structure is useless if you can't change it. You do want to 'change' the data. Immutable structures let you take a data structure and 'modify' it

Because a data structure is useless if you can't change it. You do want to 'change' the data. Immutable structures let you take a data structure and 'modify' it, getting another structure. Anything with a pointer to the first one has a guarantee that it won't ever change. With C-style language structures immutable structures wouldn't be much use. With Clojure, for example, immutability is one of a few mutually complementary features of the language in which it makes perfect sense. Immutability doesn't just mean 'doesn't change'.