iOS and Xbox 360: While you're going to want a controller to actually play games, a tablet can often be a better way to control a media centre. When you're watching videos, playing music, or using your Xbox more as a media centre, you can employ your iPhone, iPad or iPod touch to take control.
On top of being an alternative media centre controller, the app allows you to change your avatar, check achievements, and more. It won't allow you to actually play games, but chances are you wouldn't want to use a touchscreen for that. If you've got an iDevice at the ready more often than a game controller, just grab the free app and connect it to your Xbox.