I can think of two good reasons: 1. cultural expecations. consoles have controller gizmos. people expect it. its a different experience than just running an HD

I can think of two good reasons: 1. cultural expecations. consoles have controller gizmos. people expect it. its a different experience than just running an HDMI cable from your PC to a TV and calling it a day. Its a console. It lives next to your TV all the time. Its supposed to have a controller, because Nintendo's NES had one back in 1985. Cultural expectations and traditions are a powerful force. 2. there are many games that are actually superior to play with a controller than with a mouse and keyboard. Platformer games come to mind. Thats a major genre of consoles and always has been, and alot of indie game companies are exploring the genre again on the steam platform. At the moment I have to use an XBox controller plugged in to my PC to play. If Valve offered their own controller I'd gladly use it instead (assuming its good).