Why use downloadable versions of books and CDs?
Lots of reasons:
Price - downloadable versions are generally less expensive. No printing cost for the artist means a lower purchase price for you!
Convenience - a lot of players are getting away from "hard copy" books, and moving towards digital versions of their songbooks and instruction books. Less to carry to jams, less to store at home...
Speed - order now, and use the book on your tablet or computer in just a few minutes!
Quality - your digital download is the same quality as the "original" that the artist used to send to the printing company. The quality never changes over time (unlike a book which can get worn out with use).
You can probably think of more reasons that digital books and downloadable CDs make sense for you - but these should be MORE than enough reasons to help you make up your mind!