Most vertical propane smokers have a problem obtaining low chamber temperatures (225°f). The best way to fix this is to add the "Needle Valve Mod". The cheap way for the factory to do it is to have the water pan. The energy required to convert 212 degree water to 212 degree water vapor is absorbed from the burner and air around the water. This reduces the chambers temperature. If you do use the water pan to help control temperatures, may I make the following suggestion:
Never put cold water in the water pan. This will cause large, fairly long, temperature swings in chamber temp. This requires heating the water up to 212 deg. before the majority of the evaporation starts. If you use hot (almost boiling) water, the process runs as close to steady state as possible with opening the door and adding stuff. I put an old tea kettle/pan full of cold water on the "sauce warming rack" to warm the water up. This is a rack that sits above the exhaust hole of the smoking chamber. This heats the water up while the last pan full is evaporating. One less thing to mess with at 3 am.