Answer:
A multivitamin is listed as a supplement, because that's exactly its purpose - it's a supplement to your diet. People should typically consume a certain amount of certain vitamins and minerals each day in order to maintain proper health. However, the average diet today leaves many people deficient in many nutrients. Deficiencies can be caused by diets (for example, vegetarians may not get the iron their body requires due to the absence of meats) or genetics.
Even with a solid diet, a vitamin is a great asset to your lifestyle. It assures you that your body has everything it should need, and you tend to feel more energized on it. Today, vitamins have become even more specific, with certain ones being meant for men, women, children, adults over 50, etc. These vitamins cater more to individuals' needs. For example, men's vitamins will typically have ingredients which are believed to prevent hair loss, and support prostate health.