We have created a pixel buffer, sent it to the server, and attached it to a surface through which we can allegedly present it to the user. However, one crucial piece is missing to imbue the surface with meaning: its role.
There are a lot of different situations where a pixel buffer might be presented to the user, and each calls for different semantics. Some examples include application windows, sure, but others include a cursor image or your desktop wallpaper. To contrast the semantics of application windows with cursors, consider that your cursor cannot be minimized, and application windows should not be stuck to the mouse. For this reason, roles provide another layer of abstraction which allows you to assign the appropriate semantics to the surface.
The role you're probably dying to assign to it, 6 chapters in, is an application window. The following chapter introduces the mechanism by which this is achieved: XDG shell.