Vertex positions are perhaps the most important part of a vertex's information. The only data that a vertex shader must produce is the computation of a clip-space vertex position; everything else is user-defined.
Computing proper vertex positions can turn an assemblage of triangles into something that resembles a real object. This section of the book will detail how to make objects move around, as well as presenting them as a three-dimensional object. It covers how to manipulate vertex positions through a series of spaces, to allow for concepts like a change of viewer orientation. And it covers how to position and orient objects arbitrarily, to achieve many different kinds of movement and animation.