You can do other things with the water. People often use the water from their sous vide bags to wash or soak their dishes at the end of the meal if the water is clean and the bags don’t leak. People also often let the water cool down and then water their plants. If you don’t plan to use the sous vide cooker often or for a long time, you don’t need the cover. But it is very important for dishes that take a long time to cook.