Eavesdropping attacks are often easy to launch, but most people don't worry about them in their applications. Instead, they tend to worry about what malicious things can be done on the machine on which the application is running. Most people are far more worried about active attacks than they about passive attacks.
Pretty much every active attack out there is the result of some kind of input from an attacker. Secure programming is largely about making sure that inputs from bad people do not do bad things. Indeed, most of this book addresses how to deal with malicious inputs. For example, cryptography and a strong authentication protocol can help prevent attackers from capturing someone else's login credentials and sending those credentials as input to the program.
If this entire book focuses primarily on preventing malicious inputs, why do we have a chapter specifically devoted to this topic? It's because this chapter is about one important class of defensive techniques: input validation.
In this chapter, we assume that people are connected to our software, and that some of them may send malicious data (even if we think there is a trusted client on the other end). One question we really care about is this: "What does our application do with that data?" In particular, does the program take data that should be untrusted and do something potentially security-critical with it? More importantly, can any untrusted data be used to manipulate the application or the underlying system in a way that has security implications?