After thinking about this some more, and using twitch more, the main reason that I can think of as to why someone would sub/donate to somebody is if they're trying to get in the streaming game their self.
Basically, the consistent supporters of big streamers might get a co-sign from them if they support them enough. And with twitch's hosting feature, all it can take is for one of the big names to host a lesser known streamer and a lot of people will get exposed to them and that alone can get the small guy people following them/subbing to them. I've peeped this with Destiny personally, and from this standpoint it makes sense.