Two reasons to wash - first you don't know where it's been and what it has been treated with. Dead mice have been found in fabric bolts, plus the fabrics are treated with chemicals and I want to remove those. Second - shrinkage. I forgot to wash fabrics for one quilt and when I pressed the blocks a Robert Kaufman fabric shrunk so badly it distorted the block. I guess to remove excess dye, too, but I haven't found too much color transfer to my Color Catchers even with the load of dark batiks I did last week.