It allows the designer to reside in a remote location such as Santa Fe, and not have to be in close physical proximity with producers or clients. It downloads much faster, too. The chapter on design principles stresses the four rules of alignment , proximity , repetition , and contrast . Now these are ideas that I find hard to absorb just by having them stated. It ties pages together so the viewer can see a common theme.
Proximity just means keeping related things close to each other.
This dictate is illustrated by comparative examples of Web pages with somewhat random alignment, and the same page aligned following their recommendations. And it makes sense to write a book mostly for those who will be designing to convince someone to buy a product or a service. But I don't agree with their strictness, especially if you are designing something a little different. I'd recommend a little experimentation. The chapter on How to Recognize Good and Bad Design has many illustrations taken from the Web, with details changed to protect the guilty.