The simplest answer is because most people eat a poor diet. Studies show that the average diet in the US consists of too much fat, too much sugar, not enough fiber and not enough fresh fruits and vegetables. We are eating plenty of calories, but they are often "empty" calories, meaning that they have very low levels of the essential vitamins and minerals we need to stay healthy. Since the majority of meals in the US are currently eaten in restaurants or "on the run" it makes good sense to take a vitamin supplement to fill in the gaps in our diet. While it is important to remember the value of a nutritional supplement in maintaining health, we should never think that taking a supplement replaces a good diet. In other words, the first and by far most important step is to improve the quality of our diet, and then take a supplement as a nutritional insurance policy to make sure we get all we need.