There are blenders or particularly rich batiks I would buy by the bolt, or backings. I regularly buy Moda's excellent muslin that way. I suspect, however, that some people would do it because they are making a lot (maybe charity baby quilts) and don't mind doing the same thing over and over. On a good sale, that's another thing. DH bought me a full bolt of railroad denim one day at 50 cents a yard, and some lovely rich all wool plaid another time. I got tired of both, but used all of it and was happy to have the fabric on hand.