We depend on hashing, encryption, and digital signatures every day. Of these three, encryption typically steals the show. It gets more attention at conferences, in lecture halls, and from mainstream media. Programmers are generally more interested in learning about it as well.
This first part of the book repeatedly demonstrates why hashing and digital signatures are as vital as encryption. Moreover, the subsequent parts of the book demonstrate the importance of all three. Therefore, chapters 2 through 6 are useful by themselves, but they also help you understand many of the later chapters.