On twitch there's a points system. You get points for following, viewing time, hosting the stream, gifting subs or bits (basically donations or superchats on youtube) and points can manually be added every time there's a big win (over 100x or 500x). The idea is that streamers set up a store and viewers can use their points to buy things. What's on the store is up to the streamer. They can put up things with no financial value like a shoutout, slot pick etc or they can put physical items up or cash prizes.
If you're viewing on twitch then you can start building your points up but at the moment the store is not ready. Hopefully it will be ready soon and you'll be able to earn yourself a signed Jamie poster and lots of other things. And I say "soon" but Josh first said that in 1987 so who knows when that will be.