While some cage lighting is purely decorative, often it is required for the proper care and maintenance of many species. All reptiles and amphibians exhibit some form of cold-bloodedness, some mix of poikilothermy, ectothermy, and bradymetabolism. This means that most reptiles have limited physiological means of keeping the body temperature constant, and often rely on external sources of heat. Often reptiles will bask in strong sunlight to regulate their body temperatures.
Some reptiles, including many lizards, require the UV-A and UV-B sunlight provides to properly metabolize calcium and they also extract vitamins from the light. The amount of light, and kind of light required varies dramatically by species and proper research into the requirements will yield healthier and more robust animals.