Manufacturers use aluminum and other metals to make these bags oxygen proof. Oxygen is an enemy of food products. It reacts with food and makes it stale. It also contributes to the growth of aerobic bacteria, which thrive in an atmosphere of oxygen. Mylar bags make an effective barrier against oxygen.
Food grade Mylar bags are an expensive packaging medium. They are mostly used to preserve expensive food items for years on end. Some of them are resealable pouches. This ability makes them very useful to domestic consumers who want stability with usability.