The modern dust jacket came into common use by the second half of the 19th century. Early dust jackets were intended primarily to protect the bindings of hardcover books, which were the decorative center-pieces of the books of the time, and were usually disposed of before sale. As economics of book publishing shifted in the first part of the 20th century, book publishers moved the aesthetic highlights from the binding to the dust jacket.
Modern hardcovers typically have paper bindings (perhaps with quarter cloth on the spine) that offer little decorative interest beyond simple lettering of the title, author, and publisher on the spine. The dust jackets are what give hardcovers visual interest on a shelf.
Unfortunately, paper dust jackets are fragile. Dust jackets left on the book while reading are easy to tear, dent, or otherwise damage unless you're careful in handling them. Some readers remove them and the jackets often either get lost, disposed of, or damaged by creases while pressed between books in a stack.
The solution to this is cover your dust jackets with a polyester cover, which prevents both mechanical (rips and tears) and UV (fading and discoloration) damage to the cover.