honestly, the biggest advantage Steam offers me are:
1) Making my library available on whatever computer I am using (provided the games support it), without me having to drag around an external drive of some sort.
2) Keeping my downloaded games up to date, and the saved games synchronized between computers.
Granted I'm probably an abnormal user in that I have a desktop at home and a laptop I use for travel/work, but it makes it easy to keep things in sync and start a game on one, then continue on the other, with very little effort on my part.
Didn't sign up to Steam for the longest time, but recently did to try a game a friend sent me. Its much less intrusive and user friendly than I had expected.