It's been so long since I've used either of these, I had to refresh my memory with a google search. Yes they are for giving a more polished look to hems and seams. Older patterns called for them before zig zag machines and sergers were a household item. The lacy ones are for working with curves same as you would with a bias binding. You sew down both edges of the binding and encompass the unfinished seam. You stitch the hem facing onto the raw edge of the hem and then fold the hem up to the length you want and stitch (usually by hand) the other edge of the facing down. I've never interchanged the binding and facings. Hems often have a slight curve to them so the lace has some give and lies flat where the binding has no stretch and will pucker.