The most important need for a man - water. There is no life without water. Flask provide ability to keep water during Raids, Warfare or just trips.
Kettles created for water boiling, for example, for tea and for food cooking. Each soldier keep kettle during warfare.
A soldier can do without sleep for a while, without food, without rest, but he cannot do without the most important thing - water. Dehydrated soldiers - bad soldiers. To solve this problem, invented hydrators - a compact flask of water on the back, with a a small tube. Want a drink? Simply insert the straw into your mouth and squeeze the valve.
Different spare parts for hydrators and flasks.