This title is a bit misleading: they are not backing up digital books on paper. According to the article, they are instead keeping original copies of the books rather than destroying them.
Considering that we are pretty much going digital now these types of repositories will become invaluable in a few hundred years if not earlier. Thankfully a lot of people love their books and will not dispose of them even if they could digitize them. So even if a book could not be found in a repository there is a good chance that somebody around the world will own a copy.
