The difference is the experience. If they can provide an experience that people find valuable and an improvement over, for them at least, just hooking up a PC to a TV then it could be advantageous.
Forget for a moment that this is "just" a linux pc running steam. Imagine for a second that this is some company launching a brand new game console. In much the same way that Sony entered the market with the PS1 or Microsoft with the XBOX. Does the installed base of games matter? Not if the console maker can drive an initial set of games on launch that are compelling enough to drive people to buy the console in large numbers, and then the installed base of consoles will incentivize other games makers to target the console. Valve has the advantage here that they are already a top tier game studio and they already have quite cozy relations with a lot of game makers and publishers.
Will it work? Maybe, maybe not, it's hard to say just now. Ultimately the market will decide. Personally I'd give it just as much chance as the WiiU, if not more, of being a major player in the console games market in 5 years.
From what I remember the big killer apps for the PS1 ,XBOX and N64 at launch were FF7, Goldeneye and Halo respectively. The important thing being that these were platform exclusive titles.
I know plenty of people who bought an N64 for goldeneye and mario kart alone and didn't buy any other titles.
Of course Valve does have an atomic bomb in the form of HL3.
Assuming they decide not to release a Windows version of HL3 (a gutsy ploy it itself, would piss off huge numbers of gamers) they would still be competing against any manufacturer who decided to sell a Linux based gaming computer. Though perhaps selling hardware is not something they are interested in from a revenue point of view and they just want gamers off Windows and onto something without a competing app store?
