I don't understand why you wouldn't just play these games on your pc? If you went through all the trouble to build a nice pc why would you willingly play the games on a lesser piece of hardware when your pc is ostensibly a max of 100 feet away from you?
This has been responded to so many times already. Not everyone wants to be at a desk all the time. Some of us have wives, girlfriends or kids. This can let you play pc games on the sofa with the tv on. For some of us, it's potentially a game changer.