I have bought some LucasArts titles from Steam, because those aren't available elsewhere (or at least those options aren't available in my region), but I wonder why people buy games from Steam?
Sure I understand if you prefer downloadable version instead of DVD, but in many cases you can buy downloadable directly from developer (TTG is a fine example) without any third party involvement. And often there is some bonus stuff if you buy directly from developer, like TTG's option for DVD version when seasons are finished. (It's also worth mentioning that for Euro gamers it's often much cheaper to buy directly from the US developer than from Steam.)
So why Steam? To me it seems that in many cases it isn't best deal you can make.