Counter-Strike: Global Offensive
When this iteration of Counter Strike was first released, it had nowhere near the amount of players as it does today. This is mostly because of the game originally having a terrible reception, due to Valve changing movement mechanics that players of previous versions of the game had grown accustomed to, along with developing it first on consoles. The community was small, tournaments would only get a few thousand viewers if that, and there were no weapon skins.
When Valve started updating the game with weapon skins, fixing gameplay issues, and started implementing the huge major tournaments, Twitch was able to bring the game from a small niche to a household name. Two kinds of streams in particular made the game huge: the aforementioned tournaments and gambling streams. With gambling streams, streamers would broadcast themselves betting skins on a variety of websites made exclusively for that “currency.” However, the amount of viewers that attended these kinds of streams had greatly decreased within the past year due to Valve cracking down on websites that would inadvertently allow minors to gamble.
With these kinds of streams fading, thousands of viewers can now be seen tuning into streams of their favorite professional players to pick up on tips and tricks to help improve their skills.