Occasionally I'll watch a playthrough of a game I'm interested in something about (the plot or aesthetic), but don't think I'd enjoy playing.
Mostly, though, I watch like the Drawfee stream where they do improv comedy and draw audience suggestions. I'd almost rather watch a vod of a game I'm curious about, but comedy streams with audience participation benefit from seeing them live.
I think a lot of streamers are basically amateur comedians doing bits with something like a videogame to give them material. mst3k-like.
Others will play a game early or right away and be able to review as you watch. They'll say like the controls feel sloppy or the theme is grating and you'll experience that with them. Not a big deal for most people, but for highly anticipated games people are excited about, it helps give a sense of whether a game is worth playing for them in a slightly different way than a written or recorded review.
Watching people play a dnd game live gives you the energy in the chat while watching, which can make more exciting or interesting the play (do people in chat who know dnd think what's happening was a good idea? is everyone freaking out at a roll? is there a person who explains things in there which helps you understand for your own game?) It also protects you from getting spoiled when something dramatic happens.
There are people who play music or paint or work on a skill on stream, and that comes with a touch of the fun of watching a live performance. There's some extra excitement when you see stuff in real time and they could fuck up or they could do something amazing.