There are basic questions about live streaming's popularity. Why bother watching someone else play a game, especially when one could play it ones self?
Much of the reasoning is the same as with Webcam modelling.
Developers present something entirely inauthentic, via non-gameplay trailers, non-gameplay cutscenes or a sort of "rigged gameplay".
Users don't get to see the interface being properly used, and can't gauge the actual experience of playing the game.
Something like a sports-caster, or a director's commentary on a movie, the player commentating over their gameplay enhances it.
Just about any game where the person playing can comment on what he's doing. Watching a stream is literally watching a movie with director's commentary. So the more engaged the streamer is and the more interesting the game, the better the stream. -- Shammwoww
Shooting the shit with someone. Either they are popular, so there is excitement in that, or they are smart, so there is usefulness in that.
Being a member of the audience is an enhancement, but so is being able to participate in a wide community.
The connection between an audience member and the streamer can become quite close, as evidenced by donations and subscriptions.
There is some interesting sales-psychology related to small payments "hooking" a person or otherwise opening the door for larger sales. Some version of that, some version of helping another, fuels what one would otherwise think is a thankless, free, experience.