No advantages? I've dumped all my discs and repurchased my games on steam because of its convenience.
It puts everything in one place. Multiplayer,friends,guides,game forums,etc.
With steam I can do a clean windows install in about 20 min with all 150+ of my games reassociated with my OS. This would take days before steam reinstalling all my games. I re-install every 6 months so this is huge for me.
Automatic updates, ( no more searching for latest patches) downloading includes dlc,s without the need to install seperatly,cloud saving is usefull IMO, especially when reinstalling.