It's the money side of things IMO (for Valve that is), hence they sell the system for other developers. I didn't even know what Steam was until 2005 when they switched WON off -- I owned Day of Defeat retail, and played against people with Steam (I had no idea what they were on about). Also how does no-cd, no case, no transport, no sales overhead and no middleman make the game more expensive than retail (or at least a fraction cheaper).
On the other side of things, it is nice to have a system that is uniform, forcibly up-to-date. That is it's harder, (still easy) for hacks to exist. Not to mention "Anti-steam" programs (emulators) are really forced to keep changing. I don't use them, but I've kept an eye on them -- rather interesting! And the source is a good way to learn about stuff. I especially like the Steam surveys of the hardware specs of gamers, makes me feel good for having a bad PC
Although I tend to avoid games just because they're on Steam, mostly because paying for them is hard and they think they can take my PC over.
