Because it's open source and based on the Linux kernel. It's owned by them but you can do what you want with it. You can't with Windows.
So if a game works on the Steam OS, it works on pretty much any distro
I game with Steam on Linux, but I'm not using Steam OS
Also, that means that every effort made by Valve to improve compatibility is beneficial to everyone.
Edit: Also, even if it were closed source, I think it would still be good as it gives us alternatives to Windows. But
Because it's open source and based on the Linux kernel. It's owned by them but you can do what you want with it. You can't with Windows.
So if a game works on the Steam OS, it works on pretty much any distro
I game with Steam on Linux, but I'm not using Steam OS
Also, that means that every effort made by Valve to improve compatibility is beneficial to everyone.
Edit: Also, even if it were closed source, I think it would still be good as it gives us alternatives to Windows. But