I know a few people that have multiple screens and watch Twitch while playing a game. Some people use it as background noise, some people actively engage in the chat and community and some people use it to see what new games are like before they buy. YouTube clips can be edited, Twitch is a raw stream of gameplay which can be a more honest representation.
I don't watch too often but I see why other people like it.
I don't watch too often but I see why other people like it.