For an ex Microsoftie, Gabe Newell has some serious stones. Not only is he vocal in the fact that Windows 8 may be a complete and utter disaster but he has the resources at his disposal to move his entire business over to Linux in a way that benefits gamers, the game developers and the linux community at large.
The bug fixes and optimization in the graphics stack helps everyone in the community with better performing apps and UI. When profiling the performance of games it can help find places in the linux kernel that are not as fast as they need to be so Valve can write kernel patches to speed up the games, meaning once those patches are put into the kernel tree the whole community benefits from a faster kernel.
The game developers win because through Steam, they can finally release games under Linux with a DRM stack that has worked out to be acceptable to both the developers and gamers, historically these types of DRM under Linux has been met with disdain from the linux community and their "open source" everything mantra. I appreciate the ideal of Open Source and I work with it a lot and have even released my code under the GPL in the past but some things need to remain closed. The great thing with games though is some of the better shops (iD software being the poster child here) GPL their game engines once they are no longer current and all the licensees of the technology no longer have active titles based on that engine. So the community does get access to the source just not right away.
Gamers win because they won't have to deal with the abomination that is Windows 8, they get access to a faster, less bloated OS that doesn't have to support large amounts of legacy cruft, the OS is free to use, and all those other Linux ideals that everyone holds to their hearts.
Steam on Linux may just push Linux over the edge into finally making some headway in the desktop space.