Welcome to the board,
From all the books I read, mopping gives meat moisture and flavor. The mopping process started with using smokers that didn’t have a water pans in them. It added the required moisture to the meat without having to use the water pan. Later people started noticing that it also imparted a nice flavor to the food so they started developing different recipes. If you’re using a water smoker, then mopping is optional for you, but not a must. If you want that extra flavor, then go ahead and use it.