It is more engaging than television, but less distracting than videogames.
It's a form of live entertainment, like talk radio or standup comedy, it is more immediate than TV and movies. Many lesser-known streamers are genuinely very, very funny. By watching you are supporting an independent artist, putting on a show for you.
It allows me to preview games for free, that I am considering buying, or that I am interested in but don't want to to purchase.
I think this form of interactive entertainment is the future (there is a technical term for it that is escaping me). Day by day, you can see Twitch viewer numbers grow, almost exponentially. If you like games, it is objectively awesome.