It's debatable ... But when it's raining too much here and I cannot get my tortoises sunlight for at least an hour then I pop on the UVB and make sure they get a minimum (without a hiding place) of 30 minutes. Then I put their hiding hole back in place, there heat lights back on. And I feel like a better person knowing I did one or the other to guarantee a good healthy long life.... But it's for grow and digestion as well as nutrients. So yes you need one unless you live in CA or AZ where they need shade instead ( a lot ) ;-)
Some people never use them. But get their torts outside during the summer months. I live in Chicago, we have winters. I use one. When he gets outside two or three days a week in the summer, I won't be using it until winter.
Tyrs4u, you should always give them shade to get into. They know if they need it or not. Also the uv will filter through most shaded areas. At least have a few plants they could get under or behind for a little shade. They should also have access to water at that time. Over heating will kill. Just saying.