Many vitamins and minerals are considered "essential" nutrients, meaning our bodies don't make them, so they must be obtained from either the foods we eat or from supplementation. These nutrients are vitally important to our health (and life), serving as the basis for proper brain function, muscular contraction, fluid balance, and energy production.
Everybody can benefit from taking a multi-vitamin. It's will make sure that you cover all of your most basic vitamin needs.
Animal Pak is pretty much known by everyone in the bodybuilding community as the most complete multi-vitamin. It is also one of the most expensive, but many people still use it and won't settle for anything less.