From Gabe's mouth (http://www.pcgamer.com/we-ask-gabe-newell-about-piracy-drm-and-episode-three/):
Gabe Newell: The point is that there's this market that you shouldn't waste your time on, that went from, “You shouldn't waste our time on it, they'll just pirate it,” to “it's actually a really large market for us now,” once you actually do the things that allow your product to be played. And that's why some of the DRM approaches are so bad, because they create negative value, not positive value. I've had this problem with software, where my machine crashes and I wasn't able to release my license. So I have high-end CAD software that I have for hobbies, and my machine crashes and now I'm screwed because of their DRM solution. And that's bad because it's much harder to justify purchasing software that might just magically disappear and create a huge hassle for you to recover. What you want to do is go the other way, and say, “Anywhere in the world, any time, you can get your software.” It's even better if you can get it to run on more platforms, which is why Steam Play is cool, so I can buy it on a Mac and play it on a PC and vice versa. That's a good thing, that moves customers in the direction of thinking, “Oh, my content is more valuable.”
In my eyes, I often buy on steam rather than pirate because its easier and worth it. Not only do I get the game anywhere but steam helps manage content pushes from the developers, both bug fixes and new features. That's AWESOME. I also suspect that the DRM helps steam offer its famous sales and encourages small dev shops to use the platform, but that's more a theory on my part than a fact.
Quick google shows a few articles discussing it: http://www.gamasutra.com/blogs/JoshBycer/20130508/191953/The_DRM_Distinction_of_Steams_Success.php