Amoung the usual reasons to prewash, I also do it to preserve the fabric. I have been known to keep fabrics a long time and removing the sizing will keep it nicer, longer. I have had older, unused fabrics discolor and have seen insect damage where they have feasted on the starchy sizing. Have never had any trouble after I started washing the fabrics. So, the stash gets washed before it gets shelved. When I use it I press and starch as needed.