A good multi-vitamin/mineral nutritional supplement is considered "dietary insurance" to be certain essential nutrients are being consumed on a daily basis. 

6 Reasons Why You Need  A Multi-Vitamin

#1  Depleted Soil

Industrial agriculture has depleted the nutrient content of soil.  Soil nutrient depletion results in less nutritious vegetables, fruits, and other plant foods.  Studies show that plants grown with modern fertilizer can contain only 25% of the micro-nutrients of plants grown using more traditional farming methods.

#2  Food starts to lose nutrients as soon as it is harvested. 

Modern harvesting, shipping, processing and storage techniques degrade the nutrient content of food.  The long distances that most produce travels before reaching our plates causes significant declines in nutrient levels.  Furthermore, when produce is picked out of season, the nutrient content is significantly decreased compared to in-season produce.

#3  Pesticides, Herbicides, and Pollutants

Pesticides, herbicides and chemicals found in the modern food supply are combined with low-quality water, environmental contaminants from elements like degraded plastic and airborne pollutants like carbon monoxide, lead and mercury. These synergistic factors vastly increase your need for extra vitamins, minerals and nutrients to combat the formation of free radicals and the attack on your metabolism and immune system.

#4  Athletes need higher amounts of nutrients

The amount of extra oxygen and energy used by athletes requires far more than the nutritional recommended daily allowance (RDA) of the average population. Consuming only the stated RDA can actually limit your athletic performance. So if you are a competitive athlete training multiple times a week, your nutritional requirements mean you need to take supplements.

#5  People are finicky 

Most people are very insistent on what they will and will not eat, especially kids.  Trying to get kids, adolescents, and even adults, to eat enough vegetables in a day to fulfill nutrient demands can be almost impossible.     

#6  Nutrient deficiencies are extremely common in the U.S.

Concerning research indicates that one-third of Americans are at risk for at least one vitamin deficiency or anemia, despite our bountiful food supply.  One of the reasons for nutrient deficiencies in the U.S. is the widespread consumption of a nutrient-poor, processed Standard American Diet.