I've been using the Steam beta for a few weeks now, and let me just say, it's awesome. This is EXACTLY what we all have wanted, and what Linux needs.
OK let me back up a little. If you're a gamer who also uses Linux, then this is what you need. Linux doesn't need it, but if you want the convenience of playing games on your computer, then it basically does.
I know some Linux users want to take over the desktop market but that doesn't excite me personally. I don't feel the need to be using the same OS as everyone around me, or having everyone use the same OS as I do. But, I do want to be treated like a first class citizen the same way as everyone else. I don't want to go to a store to buy a peripheral and have to do three hours of research to see if something works on my OS. And I know it's not that hard to make it work on other OSes because the kernel development team (of volunteers) do it all the time, without any support from the manufacturers.
The point is that Steam will make Linux a more significant OS to companies, ideally. And that, in addition to being a gamer, is why I support it.