Hey, you know all that money we make off you guys? We thought we'd throw you a bone...an itty bitty tiny 5% bone. Thanks for all your hard work. Love, Twitch!
It works something like this: Affiliates stream games available from Twitch Games Commerce, their viewers purchase those games or related in-game items, and Twitch gives the affiliates 5% of those revenues. The idea is that streamers ought to be rewarded for what basically amounts to free marketing while also encouraging people to purchase games from Twitch instead of better-established marketplaces like Steam or GOG.