To keep it short - add functionality to game when you stream it on platforms such as Twitch, people (viewers) can use commands in chat to trigger a predefined event (spawn additional waves, spawn boss, temporarily reduce player power etc...). Streamers that play the game can pick events from an existing pool and decide how viewers can trigger it (for example use twitch channel points for simpler event such as extra waves or paid currency for a larger/harder one such as boss spawn). This level of interaction with the viewers would add additional entertainment and streaming value to the game, making it interesting to watch for longer periods of time. It also promotes engagement with the audience instead of just passively watching the stream. Viewers will often use events in attempt to "sabotage" the streamer or helping them level/get better score faster, which could lead to entertaining situations as well as extra income for streamers.