While you can get your nutrients from food sources, often those food sources do not contain all the vitamins and minerals you want, so trying to hit all the right foods can be a headache. It's incredibly difficult to get every single nutrient you need in a few foods, especially when on a reduced calorie diet. A multivitamin will put your worries to rest.
You should still focus on eating plenty of fresh fruits and vegetables, but in addition to that, a multivitamin is fantastic for mineral support. Keep your body working with a multivitamin when you're trying to pack on muscle, perform during a sport, stay healthy, or lose fat!