When I was a kid, it was NES and then Sega. You'd go over to your friend's house and would take turns playing Mario, Ninja Gaiden, Contra, etc.
When you weren't playing, you were watching your friend play. Sure, some games were multi-player, but not all of them.
Let's plays capture that same feeling of watching your friend play the game.
Watching people play is like watching sports: it's enjoyable the better the player is; it's boring if they're just fooling around with the controls. Also, like sports, the enjoyment improves the more you know about the game being played.
Twitch wouldn't be fun to watch if everyone sucked. If I am watching someone on twitch who I think is the best at the game, and I found out some other gamer X is better, I will watch that better player if all other factors are equal.
The lets plays are fun for me , same with twitch. Sometimes i cant fully get into a game so i hop onto twitch, or i'll start up a lets play series. Watch 30 minutes everyday on lunch break and after a few weeks/months you are done.
I mainly do lets play to relive my first experience in dark souls & bloodborne haha.
In a nutshell, that describes my fears and reservations about the way the current age is going. It's all watching and commenting and not enough back and forth interaction and doing.
"Viewers or listeners come to feel and consider media personalities almost as friends."
For a technical equivalent to this, look at the EEVBlog forums. It's arguably the premiere forum online for hobby electronics discussion, but a lot of it also revolves around the EEVBlog YouTube videos. There are some people who don't watch, but they are in the minority.