I think Valve are getting games onto mac and linux in anticipation of an xBox store, or a locked down version of Win 10 with an xBox store coming in the future.
The original impetus for developing their own hardware and Linux-based OS was because of the imaginable threat of Microsoft locking down Windows in a post-app-store world. It's a last-ditch contingency plan as well as a way to assert their intent to Microsoft. Of course, I'm still very happy that it has the side effect of drastically improving the gaming landscape on Linux even if none of the worst-case Windows scenarios ever come to pass.