Sometimes brown bits get stuck to the bottom of the pan while cooking and the best way to get them off is to toss some water into the pan before those bits can burn. Not much, maybe like a tablespoon - it dissolves all the brown bits into a very tasty brown sauce that coats the rest of the food in the pan. It's really not complicated, but the added moisture sometimes makes the cooking take a bit longer and isn't ideal when your goal is to cook something very dry and crispy (like when frying tofu)