With the advent of streamers as a major subculture for gaming, popular platforms like Twitch began to take advantage of a new audience who were watching streams as much as they were playing their favorite games. Enter Twitch drops, a special service rolled out back in 2017. By watching streamers as they play certain games, fans can earn in-game rewards that they can redeem in their own accounts.
Popular games like Fortnite, Sea of Thieves, and the original Overwatch have implemented these drops.
Source: https://www.distractify.com/p/overwatch-2-twitch-drops