Insulin evolved to help store nutrients for times of need. It sends sugar into cells where it can be burned for energy, it sends magnesium and other nutrients into cells to be stored for future use, and it keeps fat in storage and blocks it from being burned for energy. Insulin evolved in a time when humans had very different life conditions. Our early ancestors survived on whatever food they could find, and they did not typically have food that raised their blood sugar. They often had to go for days or weeks without much food.