vitamin C enhances the immune system. Our body contains vitamin C. It's a crucial part of cell regeneration. It's not practical to eat thousands of acerola berries (one of the fruits with highest C content), especially when the system is compromised, so the most effective way of ingesting large quantities of C is through supplementation. No, it's not natural like food, but we are not getting the nutrients we need from food any longer since the soil is barren. Our requirements today are much more because of abuse (junk food, pollution, cigarrettes, drugs, chemicals...).
All animals manufacture their own vit. C but humans don't (also chimp and guinea pigs I believe). We must get it from food and supplements.