Tanks are always in need of somewhere to hide from eg predatory warplanes.
Today’s invention allows them to lurk underwater, without much additional kit to cart about.
A tank would drive into a body of water, and quickly switch off its engine. The crew would then open the breech of its main gun and use that as a way to get air from the surface -as well as being able to see above water using two wirrors attached to the muzzle and breech block.
Armoured vehicles could thus wait, undetected, underwater for significant periods of time.