You need dietary supplements, if your food doesn't contain enough vitamins or mineral nutrients. There are many reasons for this.

If you eat unbalanced - innately or during an unbalanced diet - you most certainly have a vitamin deficiency or a deficiency in mineral nutrients.

But you can suffer from deficiency symptoms even if you live a healthy life and eat food of highest quality.

What's the reason?

Please think about your lifestyle. Often you can only have a good meal in the morning and in the evening. At noon you may have time to eat in a cafeteria or a snack-bar. Canteen food doesn't have to be necessarily bad. But it has to be fast and most often cheap. Nobody thinks about vitamin-preservation. Furthermore our body can't produce certain vitamins and if it get's some, they can only be stored for a short period of time.

This is particularly true for vitamin c. Humans and some apes share the inability to produce vitamin c and to store it for a longer period of time. This puts us at a disadvantage with many other mammals. It's true that an orange has enough vitamin c for a whole day. But what no doctor tells us is that our body can't absorb all of the vitamin c and it can not store it until we eat an orange the next morning. It's also useless to eat 20 oranges in the morning. Better eat an orange every two hours.

By the way: Many  recommended daily doses are just minimal values. There's a big difference in saying:

"This amount is ideal!"

or

"Below this amount you're in great danger!"

And keep in mind: vitamin c is a great fat burner...