If you mean engagement in terms of views/ plays, then usually very little unless it's a big youtuber. The only time we've noticed a real increase is from a Twitch streamer who happened to get a big raid when she was playing our game.
Many of the big indie streamers- like ManlyBadAssHero- specialise in horror, which is why those games tend to do so well here.
I'd definitely recommend devs reach out to small Youtubers for other reasons, though. It's always good to make new friends and you can learn a lot from seeing your games being played through different eyes.