This chapter contained a lot of theory. I hope you didn't mind too much. It was necessary to lay the foundation that will allow us to share a common vocabulary and also explain why the concepts are important. You are now well aware of what purity and immutability are and you learned some tricks to spot impure functions. We also discussed how those two properties lead to something called referential transparency what the benefits are.
We also learned that, sadly, PHP does not support most of the benefits natively. However, the key takeaway is that using a functional approach reduces the cognitive burden of understanding your code, thus making it easier to read. The net benefit being that now your code will be easier to maintain and refactor and you can find and fix bugs quickly. Usually, pure functions are also easier to test, which also results in fewer bugs.
Now that we have the theoretical basis well discussed, the next chapter will focus on techniques that will help us achieve purity...