They are vital. We need vitamins in order to see the world around us, to grow, to make bones and connective tissue, to fight infections and cancer, to heal wounds, to stop from bleeding to death, and to keep our teeth from falling out. We are not self-sufficient. We depend on a steady supply from outside sources for these vital compounds. Vitamins cannot be manufactured in sufficient amounts by the body, and must be taken in from the environment.
Vitamins are also available as commercial nutritional supplements.
While I have great respect for the results of modern nutritional analysis, I have greater respect for the long-standing relationship between humans and their natural foods. Vitamins occur in foods in forms that are the easiest for the body to use, and accompanied by important related compounds. One of the challenges of parenting is to make eating these whole foods as pleasant an experience as possible. Even though many children have periods when they are quite picky eaters, most children get the minimum known requirements for vitamins from the foods they eat. Nevertheless, we live in an age of highly processed foods.