Jackets are mounted on the outside of modern stills and act as a way of heating the pot. They provide efficient and spread even heat all around the pot, making them perfect to control the distillation of spirits. Sometimes they are filled with boiling water that pumps round (like a Bain Marie), while other times this is done via steam.
Steam is the preferred option for most distilleries as it is more responsive and allows for faster temperature changes, but the cost of boilers, safety valves and more means that for the smaller operations – it’s not always a viable option.