You can learn a lot by by watching someone else fly.
For instance TheFlyingFabio on Twitch has educational streams. He teaches how to fly approach procedures and about aircraft systems. You have educational Youtube videos of course but on the stream you can ask questions like if you’re in the classroom.
A lot of people like to join in on multi-player flights with a big community, the social aspect.
I really don’t get why people don’t get why people watch streams.
I remember when this 1st launched on steam, they were streaming someone on the MSFS page, and I thought to myself wow this guy is real real bad lol flying almost the whole time in 3rd person. Some of the things I saw him do made me cringe ;p
I watch mostly for instruction, ex. Naval carrier ops, or when I want to see a specific scenery item or feature. Other than that I dont watch streaming.
I don’t watch FS streams often, except A320SimPilot from time to time to learn a procedure from someone who has first hand knowledge.
I have streamed a bit and I have found that viewers seem to like the few Vatsim G.A. flights I’ve done. I’m not really the right kind of personality for streaming so I don’t know why anyone shows up. But they sometimes do.
But they are definitely a lot more likely to hang around if I’m flying G.A and not tube liners, (I assume because FS really shines at low and slow - and the view from FL400 is just kinda dull).
Haven’t done any in about six months tho. Maybe I should. It forces me to try to be a little less sloppy 
There were a few English lads I used to watch every now and then, they had built the cockpit of a 747 into a garage.
All controls and gizmos there. Quite interesting to see the way they had all the controls hooked up to perform.
When watching a Flight Sim video I want a real professional (or better a true pilot) discussing interesting stuff about the airplane like history, technical details, cockpit features, and other interesting aspects about the plane.
The second reason are tutorial videos with detailed explaination of how to program the FMC, the autopilot, and the ILS.
The third reason are reviews about new planes I would like to buy. To see if the virtual cockpit and the overall visual fidelity will meet my standards, and to ensure that no unpleasant visual bugs will occur after the purchase.
This last aspect is a little bit difficult because 90% of all pilots stare straight ahead on the artificial horizon and the speedometer for two hours - as if the back of their head was nailed to the headrest of the seat - without ever looking around inside in the cabin and the cockpit. 
After most review videos I don´t even know what colour the seats have or how the pedals look like. That´s why it´s a little bit difficult but after some time I could find some really good reviewers like Into The Blue Simulations, Fly From Home, Jaydee, and some more.
*Knowledge - I actually learn something
*New/nice places/approaches etc
*New A/C
*Camera work: so many streamers just put it in external view and 90FOV - it looks awfull. You barely see the A/C in formation 2m away, and world/view looks distorted. Learn to use the drone cam - its much better/versatile. Buy a cheap controller and learn from TwoToneMurphy, Daiki etc - they really know how to make some amazing shots. Even I can make fly-by-cams, cool zoomed in shots etc better than many streamers who do this ‘for a living’/play 10 times more than me…
Absolutely nothing. Do not enjoy any kind of live streaming on any subject.
Nothing and even less.