Valve want to make an appliance, that is easy to connect and run popular PC games.
I can fully understand the market for this, this product isn't for the die hard PC master race, or AKA the enthusiast. Its for a gamer that wants to play PC games but not have the hassle of the OS, setting up the OS, building or spec a machine, or buying a pre built with an OS pre installed and then figure out how to install steam, then games etc..
They or Steam want gamers who normally play console games to just plug this in, turn it on and then buy and download and install a game much like you would on Sony PSN.
also, valve have done this against the future windows app store which valve will be locked out off.. I can see the market. The Alienware Steam box looks the best IMO, I for one do not see an issue with getting another owner to play the likes of Battlefield 4 or whatever on a PC, even if its like a games console, but a PC underneath..
Bit like a PS4 really if you think about it.. anyhow competition is good.