Folding the seamed edge on a filter can help it fit the dripper or brew basket better. In the case of Melitta filters, folding the bottom seam to the side means it won't block the drain holes in a dripper like the Bonmac. If you don't fold it, you have essentially tripled the filter paper over the drain holes and it will drain slooowly (and you might want that in some cases actually!) See our gallery of images below for examples of folding the seam edge of some filters. (Also the Chemex 3 cup comes as flat filter paper - different issue!)