i think it's a way to socialize. almost like couch gaming. most online games you're at your house. other people at their house. now you have a way to make it more social. not everyone can find people locally to game with.
you also need to start thinking about ways to add extra income from these social networks. some of these streamers get donations because people have way too much disposable income, they want to support someone, etc.
you can use facebook/instagram/twitter to increase your social reach. target them with ads and sell them a product. I was just looking on teespring and it doesn't make sense to me but one of the most popular campaigns is for a ducksauce tshirt, which appears to be a twitch streamer.
http://teespring.com/twitch/duckshirts
1198 tshirts were sold. profit probably around $10 per shirt. depends on your tipping point. teespring handles the shipping. that's around 12k gross minus whatever ads were ran and whatever it cost for the logo design.
just having the twitch followers cuts down on the cost of advertising a lot. plus now you have 1.2k people running around advertising your twitch stream. they could retarget those people with google ads for some game related product or whatever they stream.