The only time I bought a bolt of fabric was many years ago and everyone was using muslin for backings and the real wide fabrics weren't available here. But no more. I usually pick a different backing to go with each quilt when I make them now. I still have some wide white that I bought for backings but it sits. I am going to use some it for other things now. I still use some white for backings but it is more rare.

I only buy my quilt linning by the bolt.

I could see buying it for either background fabric or possibly for backing I wish I could afford to buy a whole bolt the prices are so high right now here it would be a fortune.

this is my world works also. If I truly love a fabric, I will use it in as many quilts as I can. Then I miss it when it's gone!

Some people want the biggest stash they can get. I personally only buy backings and nuetrals by the bolt. I love white on whites so I often buy a whole bolt. It never stays around long.

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.

I more often than not will buy 5 yards or more and already have several bolts sitting in my house that are from 8 yards to 10-15 yards on each. I buy them first and fore most cause I can and want to. I also do so when the price has it on sale as a Walmart in a neighboring town had this week for $2 to $3 a yard so I picked several bolts I liked the quality of and took all of it. It had from 8 to 10 yards on these bolts. Most of there bolts at Walmart if full is usually 10 to 15 yards but mostly 10 yards. I use it all to make quilts for homeless children and displaced children (those taken out of there homes and put in foster care). Now I have purchased a bolt at regular price of black, white, and gray to use in my modern quilt projects and got a discount from the LQS for doing so. She knocked of a couple dollars per yard which saved me quite a bit. I also like to do free form piecing into scrappy quilts or into scrappy one of a kind cut block for a special project. I like to experiment so need many solids and printed fabrics.

I used to work in a fabric department and sometime is was to good of a deal to pass up. Have bought bolts of denium and used them to make western dusters for re-enactors. And of course my sons all wanted one. Even won a blue ribbon for one of my dusters at the county fair. I was getting really good at making them. LOL

I recently bought a full bolt of white on white. If you buy by the bolt my LQS in a neighboring town will give a discount.

Most bolts of fabric have 15 yards to a bolt. If someone is making a large king size quilt, backing could take 10 yards.
Borders could be 2 1/4 yards +/- and if you want any of that same fabric in the blocks or matching pillow cases buy the whole bolt.

Sorry to repeat what others have said. Sometimes I just don't read the replys before I answer.