Very popular to not use water... But I am going to say use it.
It depends on what your going for... But there are a few things at play here.
1.) Wood releases the sweetest and least polluted smoke JUST around the range it bursts into flames. This is ~ 650-700 deg F... You can get pretty close to this with water to suck up some extra energy, but with out it your lit coals will spread until enough fuel is lit to raise temps unless you choke it down. This means dropping your fire below what is optimal. This is one of the reasons offset smokers are nice, you can give as much oxygen as needed and keep heat low with a smaller fire that you feed more often. Your not going to be constantly feeding a WSM, all your fuel is there so the fire will grow hotter then you want.
I have DEFINITELY noticed sweeter/better smelling smoke by adding water to the pan and allowing more air in to help the fire burn better (along with allowing temps more in the 250-260 range vs 220s). More fuel consumption has been the only downside.
2.) Moisture helps smoke adhere to meat... Similar to using mustard/oil/water to get more rub to adhere to something vs bouncing rub of a bone dry cut. You can spray down your meat too, and this will help... But keeping a humid smoking environment will help maintain surface moisture for longer and will give the smoke particles something to catch onto.
3.) The dryer the air is around your meat, the more the moisture on it evaporates. This can help form a heavy crust faster, then you can wrap sooner and be done... But forming a heavy crust also inhibits smoke absorption... So depending on your goals, having more humid air around your cut will help prevent evaporation, keeping the surface of the meat more moist for more smoke to adhere to, helping prevent the evaporative cooling that causes the stall, and allowing the crust to form more slowly giving more time for smoke to penetrate deeper into the cut.
Aaron Franklin insists that a water pan must be used on any smoker, and meathead discusses most of what I covered above in his article about wood/fire (the one that talks about not soaking chunks IIRC? The sweetest smelling chemicals he names are given at JUST under or above the temp that causes a standing flame... From this your wood should be within 50 degrees of having a standing flame. IMO its good to have enough air that if you are introducing much more, they will burst into flame.)