[zahlman]

Some people really do need to supplement things because of idiosyncratic metabolic issues (particularly for B12) or just not having access to enough (RDAs for vitamin D are a joke and normal Western diets can easily be out of whack in terms of omega fatty acid balance).

Vitamin C is presumably not saving any meaningful number of people from scurvy in the developed world, but it does seem to be at least a darn good placebo. There's more to health than avoiding deficiency diseases; and we're talking about typically pennies per day here anyway. A small amount compared to the food it's supplementing.