Eating a variety of foods reduces the chances of getting excessive amounts of a food that might be harmful. For example, if the grapes you eat every day happen to have a bad pesticide on them, you'll consume a higher dose than if you were to alternate grapes with bananas, oranges, and kiwi. Or, if you eat several nutrient fortified energy bars every day, you might get too much of one mineral, which could create an imbalance with other another mineral eaten in smaller amounts. Eating a variety of foods reduces the needs for supplements. By eating many types of foods, you can better consume more of the over 600 known compounds that food offers including not only the 13 known vitamins and 22 essential minerals, but also numerous other minerals, phytochemicals, fibers, and health protectors found in whole foods.
Whole foods offer more nutrients, and better absorbed nutrients, than do pills.
For example, the iron in meat is absorbed better than that in pills. The fiber in bran cereal is preferable to taking a fiber supplement or laxatives. Getting calcium from milk replaces the need for calcium supplements. Calcium aside, milk drinkers have a diet that is overall more nutrient dense compared to milk abstainers. I take a calcium supplement instead.