It's simple: Valve wants a slice of the living-room gaming market. Ignore the base level, streaming-only boxes; those only help existing Steam users do Big Picture. The middle- and upper-range SteamBoxes that actually run games natively will be x86 gaming PC's with a living-room-oriented OS (basically, a Steam console). Steam already has customers to sell it to and relationships with game makers, so it's something that will hit the ground running a good bit better than OUYA, for example.