like NC said, a water bath to place the meat in to cook. it allows you to cook for extended times without over cooking the meat. it also allows you to hit an exact or precise temperature or doneness.
once you finish it in the water you can throw it on the BBQ or pan to quickly sear it or put it in the freezer for later. it opens up a number of possibilities that where more difficult to obtain before. think brisket, no more watching the BBQ or smoker all day watching the internal temperature and putting additional coals in. with this, it frees you up to do other things. you put it in ans go to work or sleep, etc. it takes care of the temperatures and you come back after however long you want and sear it. makes the best juiciest tender meats you will ever have