Different people like them for different reasons. For me they're basically just variety streamers using a virtual avatar. Some people are really big into the idol part of it which never clicked much with me personally, but hey if they enjoy that then good for them. ¯\_(ツ)_/¯
It's similar to the appeal of cartoons to me. It's not something I enjoy instead of live action stuff, it's just another form of entertainment. It's an interesting creative medium with its own possibilities. Most of the time, it's just another streamer I listen to while I do my work.