A good way to think about it is the description of Mycroft Holmes by Sherlock Holmes;
"The conclusions of every department are passed to him, and he is the central exchange, the clearinghouse, which makes out the balance. All other men are specialists, but his specialism is omniscience. We will suppose that a minister needs information as to a point which involves the Navy, India, Canada and the bimetallic question; he could get his separate advices from various departments upon each, but only Mycroft can focus them all, and say offhand how each factor would affect the other. They began by using him as a short-cut, a convenience; now he has made himself an essential. In that great brain of his everything is pigeon-holed and can be handed out in an instant."
Authors start with a short draft that is only juice and then add spam to make it into a book. That initial juice is the summaries of chapters that you find on the last page or two. The initial part of the chapter is just fluff and examples to drill the point down because they think you need 10 examples for each concept in order to justify paying as much for a blog post as you would for a book.