Your desk is not designed for your mouse, so it may not be as consistently smooth or hard as you want it to be. You risk straining or injuring your wrist and hand if you don't have the comfort of a mouse pad, especially if you play for long hours. The surface onto which you run your mouse affects its overall performance, not to mention its longevity. A mouse pad keeps your mouse in place, gives you better control and accuracy, and protects it from dust and other debris that lands on your desk. The mouse pad will also protect your desk from scratches that the mouse can produce.