The threads twist and pull at the fabric when they get tangled up in the wash, and you have to remove them before they go into the dryer so that fabrics come out wrinkle-free. Neglect to remove the threads and you'll see oodles of accordion pleats when you take fabrics out of the dryer -- folds that aren't always easy to press flat.
It's easy to eliminate nearly all of the strings. Just clip off a little triangular section from each corner of fabrics before you wash them. Try about 1/2" or a little less. You'll still see a bit of fraying, but not enough to create bunches of knots.
Give it a try the next time you prewash your quilting fabrics -- the little snip really makes a difference.