What this book is about, is not to teach you how to program in PHP but to show you more structured way to improve what you're already doing by reusing code and building better functions that will be useful in the long term. Beyond that, working with code that is easily readable and understandable by just applying a few programming practices. This doesn't produce the most efficient code but it does allow you to completely understand what you've written before, to easily make changes at any time.
Probably most importantly providing a different aspect to how you program.
It's time to stop reinventing the wheel every time you have to build a new website. As well to provided list of commands that you are likely to use every day including how to use them and what the parameters mean.