You asserted, "If you eat a balanced diet, you shouldn’t need vitamins." I don't know your educational background, but it is a well documented medical fact that genetics come into play on how well and for how long during the lifetime vitamins and minerals are absorbed. I happen to not absorb certain vitamins as well at 66 a I did at 30. I'm thankful supplementation can make up the deficit and extra anti-oxidants can slow my going blind.
My educational background is a PhD in biochemistry/molecular biology.
The problem with trying to present a general truth, is that there is always a pathological process that makes exceptions to the truth.
Human beings did not evolve popping megavitamins; it makes utterly no sense to assume that a healthy individual should need to pop megavitamins to remain healthy, when that is not how we evolved.
Unfortunately, too many people assume that because something is true within a pathological context, it must be generally true—and that is not the case at all. Diabetics must severely limit their intake of sugar; that does not mean that everyone needs such drastic limits. With the disclaimer that I have not read any studies regarding the role of antioxidant vitamins in slowing the progress of macular degeneration, and so cannot judge their reliability, I’ll just say that because taking megadoses of these vitamins has a beneficial effect in your case, does not mean that it would be equally beneficial to people without your condition.