I think it's more of a Twitch thing. I've seen it over there for years and years and years. It's now becoming more common everywhere else but yeah it is weird when you think about it. I think the psychology behind it is sort of like an impulse buy at the grocery store. Putting low value items at checkout encourages spending so maybe the dono bar does the same i.e. "Oh if I donate just 3 dollars my favorite streamer will hit his goal".