First off, what are fan rewards? Fan rewards are something that some games, such as Rocket League, randomly give their viewers during tournaments. When you connect your game account with your Twitch account and watch the stream of the tournament, the game will randomly send your account special cosmetic items.
Rocket League has had these fan rewards in place since RLCS Season 3 in June of 2017. These fan rewards give an incentive to the casual player to actually get into the esports scene.
I know firsthand how effective this is because when Rocket League started fan rewards, I had never heard of Rocket League esports or had any desire to watch them. I had been playing for six months at the time and was still a low ranked, casual player, but when I found out that I could get free items just by watching the stream I decided to watch all three days of the RLCS lan. Ever since then, I have been invested in Rocket League esports and watch most tournaments while keeping up with every team.
I believe any games that are trying to start their esports scene should look into fan rewards. It's a great way to get all players involved and invested in esports. It gives the casual players who normally don't care about esports an incentive to watch tournaments. That boosts viewership for the game which makes their game show up higher on Twitch which gets more people watching and playing the game.