It's there to get it away from the other categories. Every category isn't just a game, there's other stuff like events, just chatting etc to cover any content that isn't specific to a single game (like a gaming related podcast for example), so playing a game isn't required you can just talk at the audience. People who make the sexual content were using those more generic non-gaming categories (as well as popular gaming categories but having the game running in a tiny box in the corner) but advertisers took issue with advertising on the sexual streams, so Twitch made ways to segregate them from the content advertisers wanted to pay for.