@Ted Clayton -

2012:
It's my impression this customizer basically adds the ability to test drive and set various settings via the Theme Preview screen before you activate a new theme. And it's not exclusive to Twenty Twelve, most obviously because Twenty Twelve has been punted to 3.5 and these customize features are going into 3.4.

Controls:
If you continue along the vein of a screen to "set it before you save it", my guess (CMIIW) is that koop is alluding to allowing plugin/theme authors the ability to integrate their own sets of settings into this screen. It effectively would save people a good bit of time setting up a new theme because they can configure settings before they decide to activate it.