The economics of publishing are such that the authors choose to add every area they can on a given topic so that people won't reject their book for another on the basis of one missing item.

As others point out, it is also a crude proxy that buyers use to evaluate the worth of the book without reading the entire thing.

Of course, there are exceptions. One of the reasons that The C Programming Language by Kernighan and Ritchie is so popular is its conciseness.