Its the way God designed us. When we were huntergatherers and we roamed the earth for the next food supply as nomads, there would be times where the winter would come in and the food supply would drop, or there would be times of famine where there just wasnt a food supply. God was smart enough to design us with a source of sugar during these times of need for food. All mammals have this food storage system called fat. When you see those commercials asking to aid children starving in other countries, those children are relying on these fat stores to survive their lack of adequate nutrients.