I've always wondered. I even checked the Wackopedia page.

And besides the reading room only had two threads, and that was making us look uncultured.

Isn't it because they keep dust off the cover of the book?

Is dust a problem for books?

I think it is if they're in a bookshop for a long time. It spoils the fine leather binding that books always has when dustjackets were first invented.

I call them tome-guards.

Or dirt-jerkins.

Detritus dinnerjacket