Providing supplements for captive reptiles is essential! In the wild, they eat natural food from the area they thrive in, which includes soil for the plants, plants for the bugs, and bugs for the reptiles. They get all the nutrients they need in the wild, but when we keep them in unnatural environments and feed them unnatural food, it's critical that they get extra vitamins and especially calcium.
Look at it this way... if you were kept in a prison cell and fed only bread and water, soon you would be malnourished and deficient in vitamins and minerals. You would need full balanced meals and supplements in order for your body to function properly.