The water can be used to help with temperature somewhat. Some folks will load up a pan with wet sand to absorb and hold heat. Personally, I don't know if it truly works or not. I also don't know how it affects the heat from the element getting to the food. And I have never bothered to experiment. I suppose if one must open a smoker continuously to spritz or baste or poke and jab, the "heat sink" might help temp recovery of the chamber. And to answer your question, the thermostat does regulate the the cooking temperature. If cooking a great large shoulder or brisket, use a remote meat thermometer to cook to your preferred internal temp, and pay no attention to how long it will take. The best advice I can give is to follow your own gut feeling: "I'm thinking I'm going to just add the little juice or liquid holder I bought with the smoker and add apple juice. Not sure why, just going to do it " Enjoy.