It adds flex to the container for variations of atmospheric pressure. It’s what they mean by the fill levels.
Milk containers are filled by weight. The exact volume of the liquid can vary slightly based upon the atmospheric pressure.
If the container has no flex the top will pop off or the sides will crush in with pressure changes say when a truck drives over a mountain pass to deliver the milk.






Taiwan “Hey Ukraine, got a few of those navel drones for us?”
Ukraine has shown that a traditional naval power is pretty weak.