Is this a repeat post? I could have sworn I was the first to respond to this thread, in fact I know i was....
I guess I hit preview post.... I'll try to answer it again. There are two cooking methods, direct and indirect heat. The water pan deflects heat and creates an ideal indirect method of cooking as it adds moisture. It also helps to stabilize the heat in your smoker. If you remove the water pan, you now have direct heat, as well as fat dripping onto your heating element.
If you don't need the moisture or hassle of the water pan due to brining, injecting or just attempting to increase the heat in the smoker, fill the pan with sterile play sand and cover it with foil. the sand absorbs heat to a point and acts like a heat sink, providing you with indirect heat.
I use sand in my smoker's pan and my BBQ still comes out moist. I generally mist my meat with a mixture or apple juice and water and don't need the hassle of filling the water pan or it's moisture.