Think of your multivitamin as low-cost nutrition insurance. Try as you may to get all your nutrients from a balanced diet, you probably fall short some days. In fact, as many as 80 percent of Americans don't eat the recommended minimum of five servings of fruits and vegetables a day. That's why some nutrition experts now recommend that all adults take a daily multivitamin.
Ref: (Fairfield + Fletcher, JAMA 2002)

Vitamins and minerals work together to keep the body growing and functioning normally. They help convert food into energy, keep cells healthy, and support the immune system. Vitamins and minerals also help your body use the proteins, carbohydrates, and fats in your diet.

Here are some of the ways that multivitamins may help:

Support immunity. Poor nutrition is an open invitation to poor health. Research suggests that getting the recommended amounts of vitamins and minerals your body needs daily is a factor in the maintenance of good health. Studies also suggest taking a daily multivitamin may help maintain your immune function.

Neutralize Free Radical Damage: Antioxidants such as beta-carotene, lycopene, and Vitamins C and E, which are also found in many daily multivitamins, are substances that help protect cells from the damage caused by unstable molecules known as free radicals.

Maintain strong bones. You probably already know that Calcium is critical for healthy bones. But did you know that recent studies confirm that Vitamin D is just as important for maintaining strong bones? Yet it's difficult to get this nutrient through diet alone. Vitamin D is produced in the skin following direct exposure to sunlight. But necessary use of sunscreen, weak winter sunlight, and poor skin absorption all work against this vital nutrient. And while Vitamin D is added to milk, many people don't drink enough dairy products to benefit. Taking a multivitamin with 400 international units (IU) of Vitamin D may help.