speaking for myself: 1. Shows my friends what I'm playing. This leads often to impromptu multiplayer in a diverse social group that never did so before. Fire up world of goo then get a couple of messages from friends who were 'away' a minute ago to ask "lets kill zombies." 2. keeps everything mostly up to date. Patches usually roll out in hours of their release and are automatically applied. 3. Steam Cloud. Settings, cd keys, savegames - all stored between my laptop and desktop without any effort. 4. Goddamn easy to buy.