99% of fabric is now milled overseas and is shipped stateside in shipping containers. I am sure it needs to be treated to deter unwanted hitchhikers. I definitely have a sensitivity to chemicals so I prefer to not use it straight off the bolt. It doesn't take long before the aroma of whatever it has been treated with starts to make me physically ill, and then there are the posts here of people who have had skin reactions. But even in the days when fabric was milled in the US so possibly not treated with insecticides and sizings it was always best practice to prewash, at least that is what I was taught back in the 70's.
The nature of the machines that mill fabric tend to stretch and distort the fibers so when the machines roll it onto a bolt I am sure it is off grain and the fold may not necessarily be on the straight of grain. Prewashing helps to relax the cotton fibers and allow them to return to the state they were in prior to the machine distorting them. This again, was more important to garment manufacture than quilting but I think it is prudent even for quilting as it helps you achieve more accurate cuts when cutting LOF.
There are more reasons to prewash then just checking for bleeders for many of us. I have cited two.