In the first part of the book, we discussed a lot of code in rather abstract terms. The reason for this approach is that code can live in a number of different places, and those locations aren't that important in the bigger concept of object-oriented programming.
However, after you get comfortable with OOP, it's important to understand what is going on from the time a request is made to your site to the time a response lands on the Web browser. This knowledge will allow you to push your application development well beyond just creating pages.