when you have immutable data structures by default you get the additional assurance that other places in the code that previously used an input to a function ca

when you have immutable data structures by default you get the additional assurance that other places in the code that previously used an input to a function can continue to work with their data without needing to worry that it might have been changed by its use by a function elsewhere, since that function cannot modify the data, only return a new 'version' (through structural-sharing)