Reptiles need UVB for biosynthesis of vitamin D, and other metabolic processes. Specifically cholecalciferol (vitamin D3), which is needed to for basic cellular / neural functioning as well as the utilization calcium for bone and egg production. The UVA wavelength is also visible to many reptiles and might play a signifiant role in their ability survive in the wild as well as visual communication between individuals. Therefore, in a typical reptile enclosure, a fluorescent UV a/b source (at the proper strength / spectrum for the species), must be available for many captive species to survive. Simple supplementation with cholecalciferol (Vitamin D3) will not be enough as there's a complete biosynthetic pathway that is "leapfrogged" (risks of possible overdoses), the intermediate molecules and metabolites also place important functions in the animals health. Natural sunlight in the right levels is always going to be superior to artificial sources, but this might be possible for keepers in different parts of the world.