The premise being you want the fabric to shrink as much as possible BEFORE using it in a quilt so that the resultant quilt shrinks less upon completion. Personally I'm not overly concerned about fabric shrinkage as I use 100% cotton batting that is NOT pre-washed. Therefore, when I wash the completed quilt it's the batting that is shrinking more so than the fabric creating the crinkled look that I love.
Then there is also the desire for any fabric bleeds happening before construction as well.