There's a few reasons why people use a mouse pad, here are a few:
Softer on the part of your wrist which rests on the desk when u use the mouse
Sometimes crumbs/similar get stuck between the mouse and the desk, which can cause damage to the desk
Mouse feet last considerable longer
Much quieter movement of mouse
More even surface, while maybe problems are not noticed while doing things like browsing forums, for gameplay and more mouse-based activities, having a mouse pad really refines mouse movements
More info here: http://mrdoob.com/projects/chromeexperiments/google-gravity/