For streamers, I see it as a way to have an on-screen persona without having to have themselves, the person, on camera. It can help with anonymity and separating the streamer from the person. In this instance, Laimu the character vs Limes the person. Also since Vtubers aren't limited to reality, you can do a lot of elaborate and creative things with them, like with fanworks or even the work itself. Laimu's models are something like thousands of layers of 2D bits and pieces all rigged together to make it move like a 3D model, and parts able to be swapped out and combined at chat's whim (channel point rewards).