It's not a good book and was basically just used to get Matt Walker on daytime talk shows. It doesn't even attempt to answer the question in its title, and it has many cherry picked studies that don't actually prove what he's implying they do.
I'd argue that regardless of the author's motive, as a layperson, it's a good book because it's easy to read and started me down the path of exploring my sleep in-depth. My take away from the book is, like so many other biological functions, we still don't really know why we sleep but we know sleep is important. Learn more if you want to, learn about why this book could be considered controversial but for most people the takeaway of "sleep good" is what matters.
The book is great and Matt Walker is one of the leaders in the field of sleep study. There are numerous studies backing what he's written - even though there were some inaccuracies with some of the citations & ways he presented some facts, that is entirely expected in a book presenting thousands of such facts, and the author is planning on fixing all in the upcoming revised edition.