Steam became amazing. It is my universal game launcher actually - even games that I have on discs or bought elsewhere (GOG, D2D, Gamersgate..) I am launching via steam.
Why?
1) Ingame overlay - friends, webbrowser, all ingame
2) Screenshots anytime anywhere with instant upload to cloud. No fussing with imageshack
3) It works reliably and is quick and, well, flawless. For me, at least.
Steam does not have monopoly, but even if it did - guess what, people made Valve successfull because Valve provides services and games that tens of millions of people WANT to use. Success deserves to be rewarded!