UVB enables birds to produce vitamin D, also known as the “sunshine vitamin,” which is essential for calcium metabolism and immune function. UVC is normally filtered out by the earth's ozone layer and does not exist on the planet naturally.
UVB plays a very important role. It allows your bird to synthesise Vitamin D3 in their skin or through a special process involving the preening gland. Parrots need Vitamin D3 to metabolise calcium which they use to produce eggs, build and maintain strong bones and maintain normal growth.
Keeping your bird under right amount of UVB needs some consultation with a Certified Professional.
Despite other benefits from UVB, it helps processing the calcuim in captivity and help improving the birds vision.