The primary function of the Wii U GamePad's touch screen during any Black Ops 2 match is simply to display the mini-map. That might not sound very useful, considering that map is also on-screen at all times, and it's easier to simply glance down at the corner of the TV than look down at the GamePad in your lap.
But the great thing about the GamePad's version of the mini-map is that you can use the touch screen to zoom in on different areas of the level. So if you're defending an objective, you can give yourself more detail to work with—you'll see exactly where each enemy is coming from. Or you can zoom in on one specific area and keep an eye on it while you go off to fight in another.