In case you've never heard of Steam or Valve, here's a little description for both: Valve is a company that makes Steam, and Steam is one of the world's largest gaming distribution platforms. So it makes sense that they'd want to expand their reach from the desktop (Windows and OS X) and go out to mobile devices as well. It also makes sense that they'd reach for one of the most popular mobile operating systems first.
In my opinion, Android really needs a new source for gaming. I love the Android Market, but since any developer can publish virtually any piece of code they throw together, I feel that Android really isn't the best platform for gaming. Granted, companies like Gameloft and EA Mobile sell HD games for Android on their websites and more and more known developers are coming to the Market (and the new Amazon Appstore) because of the incredible amount of Android sales, but having one place where all of the quality games are located could really push Android as a platform for gaming.