I wouldn't watch someone else play Fallout 3, but I like watching people play competitive games like Hearthstone and SC2 because I feel like it makes me a better player. In the case of Hearthstone, the difference between watching and playing is minimal - it's basically the same if the streamer makes the play you would, and if not then you learn something (or, occasionally, get to feel clever because they didn't see your superior play).