Someone sees a new game, they go to twitch to watch it being played - there is no need for them to subscribe to a channel when it's being seen for this common purpose. But maybe they want to tip a quick buck because it had value, and they don't have bits because they're new to Twitch or they're a casual user.

A dollar today was only 25 cents a couple decades ago, and a lot of people would happily give it if it was as easy as pulling out a dollar bill and handing it to someone. America has a strong tipping culture, and the only thing holding influencers back from a large piece of that is the ability for viewers to do it quickly and easily.