UV doses sufficient to cause erythema, mild sunburn, will trigger programmed apoptosis(programmed cell death) and if you don't have some other sensitivity to UV and do have psoriasis (another chronic autoimmune disease) after the sunburn has subsided you can get substantial reduction in the size and severity of skin lesions in many cases. The reset can sometimes last for awhile. This is well known and is the basis for natural (sun) and medically administered (uvb light) treatments.

In no way is UV caused DNA damage good for a person in any dose. The effectiveness of this treatment has nothing to do with vitamin D generation or hormonal changes as far as I know. You have to damage your skin to somehow reset the immune system and get the result. It is a bargain with the devil as I think are many radiation based therapies. That doesn't mean they aren't useful.