Thing is, if you need therapeutic doses, to fight a serious infection or worse, then you can't really get enough vitamins from food.
Obviously prevention is better than cure, and maybe you can prevent illness just from a good diet, but if in spite of this you fall ill, then sometimes we have to suspend our idealism and go for supplementation, sometimes in high doses, especially where vitamin C is concerned.
I've tried e.g. loads of oranges when going down with a cold, and it just doesn't cut it, and you may be too ill to eat.