Something needless isn't required, necessary, or wanted. If you haven't touched that plastic back scratcher since Christmas 1992, it's probably needless.
If your mom frets every time you walk out the door, you can assure her that it's needless worry — you'll be fine. And buying extremely pricey underwear might seem to be a needless expense — who cares how fancy your underwear is? Needless comes from need and its Old English root nied, "necessity."