Everything is much closer together, you don't have to move your hand only your fingers. It is clearly the better input device for pure digit sequences.
Worse, if you're on a laptop a lot then this habit is kicked out the door. Enabling a numpad on the keyboard is less efficient than just using the top row.
I'm left handed; the numpad is freakin' awesome for number entry. Back when I still played games like Descent and WoW, it was also very efficient to setup the mouse for my left hand and leave my right hand on the number pad. Aim and fire with mouse, maneuver with numpad (in Descent), or action bar items on numpad (in WoW).