Window valances
There are many different elements of interior design that seem to have been invented simply to sell more stuff. Window valances are one of these items. Since when were your curtains not enough to dress a window? These extra lengths of fabric were fitted along the top of the window, in addition to the curtains.
They serve absolutely no purpose. If you want to repurpose the fabric into some dresses for some visiting children, then go for it, your windows will be fine. Curtains or a blind are all you need to dress your windows. Choose a simple fabric to keep it modern and stylish.