UVB plays an important role in the vitamin D metabolism of many reptiles. Vitamin D allows calcium to be absorbed from their gastrointestinal tract, as well as having other important functions.
If low vitamin D levels occur, the resulting low calcium levels can result in bone density issues, poor digestive tract motility, muscle weakness, reduced immune system and neurological symptoms such as twitching and seizures. One of the most common conditions is called metabolic bone disease where their bones lose their calcium stores and become very weak and rubbery.
In general, most of the lighting recommendations for reptiles revolve around the UVB component.
UVC is generally filtered by the ozone layer and can be harmful to living cells.