It is just so tacky when the books on your shelf don't have the same aesthetic. That Penguin paperback looks so odd next to the Harlequin romance, and they both clash with the hardbound Harry Potter book you stayed up until midnight to buy when you were 12 years old. Such a fate never worried discerning 18th and early 19th century book buyers--they just made sure to get the proper binding right from the start.
Until a few decades into the 19th century, most books could be purchased sporting a simple, temporary paper wrapper or just in printing sheets, folded and side stitched. The buyer would then have a binding put on to match his or her taste and to express the value he or she placed on the book. If the book did not get bound, it usually fell apart or was badly damaged.
Rare PS744.F4 1789. For another good example see our Acts and Laws of the State of New-Hampshire from 1788.