When you try to explain a complex concept, it is generally a good idea to give a very simple and minimal example illustrating it. This is why in this book you see about a dozen different ways of calculating the sum of an array, each highlighting a certain CPU feature.
But the main purpose of this book is not to learn computer architecture just for the sake of learning it, but to acquire real-world skills in software optimization. The next two chapters exist to help you achieve this goal, as they contain detailed case studies of various algorithms that are much harder to optimize than the sum of an array.