Allows for laziness, in languages like Haskell. Also, some data structures are simply incorrect when the data is mutable. A hash table with mutable keys comes

Allows for laziness, in languages like Haskell. Also, some data structures are simply incorrect when the data is mutable. A hash table with mutable keys comes to mind. Why? Once you insert the key-value pair, and mutate the key, you may not be able to find that key anymore.