As someone who streams on a smaller scale, and who has donated a few hundred dollars to streams (spread out over time, and usually towards charity), there are a lot of different reasons and incentives to donate.
- Reaction from the streamer. Donations get their attention and will show (or read aloud) a message from the donor. It could be a joke, or just kind words of support.
- Interaction. Streamers will often set rewards for donations above $X. This could be something like "choose the next hero I play in Overwatch", or, as many Tabletop streamers do, "Player of your choice automatically rolls a Natural 20". These can also often turn into glutton for punishment rewards, like doing shots or eating hot peppers. Sometimes donations can trigger sound effects, e.g. a loud roar during a tense moment in a horror game.
- Competition. Someone else mentioned the "telethon" mentality of wanting to be the highest donor, and that very much applies. Streamers will often include on their channel the rankings of their top supporters.
- Milestones. Streamers will often set goals of how much they want to raise over the course of a day or month. This could be something like "help me afford a new CPU" or just "Keep the show going". If they hit it, they promise things like game giveaways, 24 hour streams, dyeing their hair a crazy color, etc.