It’s something that probably doesn’t apply to you – yet.
Away mode and Intel’s Instant-On technology are very similar and supported in very much the same way with Media Center PCs. If you have hardware that supports it, Away Mode allows you to do some interesting new things with the power button that weren’t possible before. For example, wouldn’t it be cool to be able to press the power button while you’re listening to a CD and it instantly shuts off? And wouldn’t it be even cooler if when you pressed the power button again it turned back on and resumed playing that CD? It’s the interaction with application(s) as well as unique power management states that makes Always On a cool addition to Media Center.
Away mode allows you to put the PC into a standby mode where the audio and video are turned off, the fans turn down, but the PC otherwise stays awake to service the recordings, your USB peripherals, and extenders. It’s an ideal power state for Media Center PCs that was previously not available. Seems simple, but if it were I guess someone would have done it by now.