Many book collectors insist on having fine condition dust jackets or covers on their books

  Generally speaking,  a hardback book's dust jacket is an asset when shopping; the price for a book is higher if it has a book jacket in fine condition.  A tear or other problem on the jacket lowers the price of the book; many used books sell with no jacket, which reduce a book's value significantly.

According to Bibio, 

 Once intended to be temporary and disposable protection for beautifully bound books, dust jackets have become–in some ways–more valuable than the books they protect.

Biblio, a book search engine, also has helpful articles on book collecting.

(https://www.biblio.com/blog/2015/04/a-brief-history-of-the-dust-jacket/)

(https://www.biblio.com/blog/2020/10/how-do-i-find-a-specific-book-cover/)