The text is a bit abstract. Disallowing mutation of values can have a lot of practical advantages, not unlike disallowing global variables. For example, strings and tuples are immutable objects in Python, and this makes it possible that they can be used in dictionaries as keys.

Here is an tutorial/example which shows how changing physical entities can be modeled in this way, with the example of describing a rocket: https://aphyr.com/posts/312-clojure-from-the-ground-up-modeling .

This has a lot in common with how things are described in physics, for example.