A bolt is anywhere from 10 to 15 yards. It takes 9 yards to back a king-size quilt. That's one reason.
I buy bolts of white on white, cream on cream and other "neutrals" occasionally. I did buy a 15 yard bolt of big red roses on a royal purple background. In my defense, my daughter told me it would be good for stackNwhack and one block wonders (and it only cost $.50 a yard). I have a bolt of a gold print that makes a great accent or blender with orientals.