Generally, collectors and librarians prefer not to find rare books damaged. In the case of assessing the artistic value of books, naturally occuring damage is helpful to get a better look at the item in question.
Here, damage to covers and spines allows closer examination of the hand-sewn binding styles of early bookbinding. Some bindings, like the Al-Bukhari, consist of pages gathered into folded folios, and then each folio is sewn directly to the folios before and after.
The most common European binding format was to use leather or linen bands. The folded folios are sewn to the band, and then the band is attached to the front and back covers. Depending on size, each book may have as few as two or as many as nine bands down the spine, visible on the outside of non-damaged books as raised bumps on the spine.
The bands visible at the top and bottom of a book when it is closed are called crowns or headbands. The headbands help protect the pages from damage as books are pulled off of shelves.
The Livy book gives an idea of just how tightly the spine leather is pulled over the bands and what it looks like when old leather begins to crack and pull away from the spine over a band. The leather has started visibly cracking only over the bands, including the bottom and now-absent top headbands; the rest of the leather in between bands is still in relatively good condition.
An additional method of strengthening a binding during this time period was to affix strips of manuscript or printed remnants to the sewn folios with adhesive. Horleder and Thomas Aquinas offer excellent examples of this practice. Manuscript remnants were also occasionally used as an added layer under end papers, as visible in the images from Tacitus.