The benefit is that Valve has access to the entire OS, allowing it to theoretically optimize it for games in ways it can't in windows, to create a truly gaming-centered OS.
It will also allow cheaper Steam Machines (no Windows license to pay for, which will also
remove Valve's dependency on Microsoft, assuming it's a success).
So is it another requirement for developers, Windows, Mac, and Steam? And more importantly will they continue to offer Steam, supporting Mac And Windows games in those OSS environments? Thanks!