Sugar is an essential ingredient in making canned jam for several reasons. First, sugar acts as a natural preservative, which helps to prevent the growth of bacteria and mold in the jam. This results in a longer shelf life for the jam. Second, sugar helps to thicken the jam by drawing out the natural pectin in the fruit. This creates a jam that is thicker and easier to spread. Finally, sugar adds sweetness to the jam, which enhances the flavor and makes it more enjoyable to eat. However, it's important to note that reducing sugar in jam recipes can alter the set and shelf life of the finished product, so it's best to follow a trusted recipe when canning jam.