In writing, form serves function. And books, which are a form of technology (albeit one we take for granted because it's been around quite awhile), utilize form to support function just like anything else.
This technology, or art, that allows us to craft books is not new. In fact, books produced during the medieval era already conformed to a well-defined set of rules that publishers today still follow.
There are a lot of conventions in book design, a vocabulary, and a set of best practices that are commonly used by professionals.
Of course, you might wonder why you, an author about to self-publish, should learn these concepts and care so much about design.