It's just really good. I still try to buy games from GOG (0 DRM), if a game's released across both distributors, but the usability and design of Steam is just really good.
They also do in house streaming, so I can play the latest blockbusters on my sh*tty 5 year old laptop while my wife watches "Strictly Come Bake Off With Me".
Did I mention the Sales? Honestly, with a bit of patience you can load up on a years worth of AAA gaming every Summer for the price of a single new PS4 release.