Valve already has Steam clients for other OS. What's the point of having their own OS?
If the Appstore model catches on in Windows (with MS deciding that they'll not allow installation of programs outside of their appstore*), then Steam would be out of business.
Personally, I see a release like this as something that may serve to get more people used to Linux. The issue right now is that Linux is unbelievably fragmented and, for the average gamer, hard to use with little software available natively. SteamOS may be able to change that.
I can't see anything good from having their own OS that they couldn't do by contributing to the Linux kernel without making their own distro.
I'm pretty sure they're going to contribute quite a few things, but the problem is that that does not automatically translate into a good gaming ecosystem based on Linux, as there is no guarantee that other distros will use these changes. Building a showcase that demonstrates what is possible is a necessary step towards getting other distros to accept and implement similar changes.
It's like Adobe suddenly deciding they would rather make their own distro just for Photoshop.
Except that's nowhere near an accurate comparison. Valve want to develop
the platform for multimedia and games, they want to make a PC-based console-workalike that combines the power and flexibility of a PC with the "it just works" ease-of-use of a console. That I believe to be a very good thing.
EDIT:
Oops, forgot the footnote I intended to place here.
*There is about a snowball's chance in the sun for this to actually happen. MS is currently in a reorganization period after the Ballmer departure; I do not expect their desperate attempts to copy Apple to continue.