The killer feature with Steam for me at the moment is a combination of,
a) some 300-400 games in my library, picked up over more than a decade, and probably 95% purchased on sales. There always seems to be a sale. This offsets the price of a computer as opposed to a console.
b) Steam Link, which lets me play those games in the living room using a regular Xbox One controller. The Link has recently migrated from being a hardware device to being software that runs on your TV (I have a Samsung TV) and now recently on iOS (iPhone/iPad/Apple TV).
Link works very well and has effectively moved me to the living room for enjoying games.
It has also made my game library available to my wife, who wasn't going to sit down in front of a computer to play games any time soon, but who has found some games she really enjoys playing by virtue of the simpler, more direct interface that is Steam's big screen mode.
While Steam the desktop app is a horrible, ancient mess, the big screen interface is surprisingly decent.