Because it's important for all authors to know, whether or not you doing it yourself. Book design is a complex art. Actually, I would highly recommend NOT doing it yourself. There is a lot of knowledge and expertise that goes into designing a good looking book and there is no way I can cover it all in one blog post.  But your still not convinced it's important for you to learn book design skills. You are a writer, after all. Won't your publisher set you up with a design team. The answer is yes.