There are lots of books on visualization that describe best practices and design concepts, but what do you do when it comes time for you to actually make something?
If you don't know how to use the software in front of you, the abstract isn't all that useful. And with growing amounts of data, it's becoming more important to be able to make sense of and communicate with it all.
Yau draws from his experience as a graduate student in statistics and his work with major news organizations for an engaging, data-first approach. After all, visualization is about the data it's based on...