I'm a bit late to the party and have just recently begun watching Twitch regularly, but I've watched let's plays on Youtube for much longer. I mostly watch games I don't have any intention to play for myself (e.g. Hearts of Iron IV) but that I find enjoyment in when someone else is doing the playing part. I also watch a lot of Super Mario Maker levels and Mario romhacks that I just don't have the skills to play myself. As of late I've also started watching more and more retro games.
Then you have people who have interesting/weird/random personalities that are entertaining to watch for those reasons. There are certain Twitchers I just listen to because of all the funny or crazy stuff they say.