Dust covers or Dust jackets are the paper or waxed paper coverings on a hardcover book that shows a synopses and an artwork somewhat relevant to the interior. Depending on the era, D.J. art could be designed by an up and coming artist. If that artist became popular, then that DJ, regardless of the book, can be very expensive. Some of the artists become popular for DJ art alone and are seen on every book in a gendre for an era. A good example of this is Boris Vallejo and Julie Bell. If these names don’t ring a bell check out the link ( https://www.borisjulie.com/ ) and you will see examples of the art that graced science fiction and fantasy covers from the 1970s-1990s. First editions of books with this art art a boon for collectors.
Sometimes publishers will reissue novels with different artists. When this is done with multi volume series, people can become frantic to obtain matching cover art.
Another thing that collectors look for on Dustcovers are mistakes. The most common of these are misspellings of the title or a type face that doesn’t match the size of the covers. But sometimes there are more drastic changes. For example a recent book in the Valdemar series by Mercedes Lackey contains a synopsis of a plot for a book which never got written. One can assume it was for an earlier draft of the novel. This print run was cancelled and new covers were printed. The old ones are to be discarded and destroyed, but in a few years they will be resurfacing.
Are there a real tangible value to these? Possibly, but the whole idea here is to pay attention to the covers. Enjoy the art, the skill it takes to design them and see what the artist has created else where.