Dietary supplements don't need to do anything, by definition. Tons of them don't. That might sound strange, since half of Americans take a vitamin or mineral supplement daily. But there are in fact, reasons to take some of them. Let's say you eat nothing but ground beef, Cheerios and Dr. Pepper every day. My nutritionist sister once saw a patient who lived this life. The human body, a machine that evolved over millions of years requiring a variety of different molecules to work best, was not optimised with a ground beef, Cheerios and Dr. Pepper-only diet in mind.