So the shift-tab thing feature was copied from a chat program called Xfire, which back in the days of AIM and such was a neat way to be able to talk to your online friends while in a full-screen game.
Steam copied that feature and added a web browser, and the rest of those interactions some time after.
I agree that it feels like marketing BS but it is cool to be able to chat with out-of-game Steam friends in-game.
I remember that for quite a long time, Xfire also did it way better than steam did.
Xfire worked quite well accross all games - they had a nice variety of skins you could install (compared to Steam's 3 skins including the default one, IIRC).
It saved your screenshots and videos in 'the cloud', had some neat 'join server' feature that worked in almost every game and it mostly worked without any issues.
You could also use their server browser so you didn't have to even launch a game beforehand, you could just browse for your servers in their program and then jump right into the game.
Seeing Xfire mentioned brought some nostalgia to me, thank you :D
(Also, Xfire changed to a tournament system and eventually died. Which is a shame but steam does probably everything and more than Xfire does nowadays. Except custom skins?)
Steam copied that feature and added a web browser, and the rest of those interactions some time after.
I agree that it feels like marketing BS but it is cool to be able to chat with out-of-game Steam friends in-game.