They don't allow you to change a lens. You screw either the wide angle or the telephoto on the end of your lens and the seller includes an adapter to allow connection to your camera's lens. Whether or not you need them or whether they are worth the money is a judgment call and others who have used them might comment about the quality of the photos taken with the adapters.
Canon EOS-M with 18-55mm Zoom, 22mm prime, 11-22mm WA lens, and Canon 55mm F1.4 with adaptor.
Olympus PEN E-PL1 with Kit zoom lens, Olympus M.Zuiko
17mm F2.8, Olympus M.Zuiko 40-150mm, Olympus 45mm F1.8, Canon FD 50mm F1.4, Olympus Zuiko 50mm F1.8, Pentax 135mm F2.5, Panasonic 20mm F1.7, Panasonic 14mm F2.5.
Panasonic DMC-GH2 with the kit lens, Panasonic 45-200, Panasonic 14-140mm, Panasonic X 14-42mm, and all of the lenses that I have for my Olympus PEN E-PL1
Panasonic HC-V700 camcorder