It's the same reason people watch sports, it's entertainment. Most people suck at games so they get to watch someone who's a lot better than them play, or someone who is funny. That's basically it. And the chat is usually just shitposting or game discussion. 

As for your question, mixer seems to be much faster. The streams load in <1 second as opposed to about 4 or 5 on twitch. Also mixer automatically shows you the past ~30 or so chat messages when you first join. That's all I've noticed, I watch a couple osrs streamers occasionally and checked out mixer.