We chat about videogames and videogame accessories.

Presumably, for the same reason you'd buy any other console: the hardware is higher-quality or lower-cost than some equivalent computer, with regards to playing games.

@Akumu, below: For one, I'm reasonably sure that Stupid Patent Or Copyright Bullshit would make designing a traditional controller inconveniently difficult. For two, Valve has been a computers company for ages, and I see this as applying principles of mouse design to controller design. The primary interaction is on the index and middle fingers, with supplementary "side" buttons on the thumb and ring finger.