My Uncle (a cooper) works in a whisky distillery making casks, and I asked him why they are that shape, rather than cubes or straight cylinders. Whisky casks are a barrel with a bulge in the middle, called the bilge. The bilge helps the cooper make the barrel watertight. But its shape is also important for moving the barrels as it means you can roll and turn the barrel easily as there is less friction than for a box or cylinder, but it can still be stacked.
In whisky making. the ““angels’ share”” is when a bit of the whisky evaporates over time — for the angels! A cask holds more whisky with less surface area than a cube or cylinder, which means you loose less whisky to the air — the ““angels’ share”“.
