for some, it's only about the cutesy anime voice and character attached to it, it's how miku got so popular in the first place. for most, it's the appeal of using software to make it sing whatever the hell you want it to sing. it's a very good tool for musicians to put their work out there without the notion that the singer is doing bad things, like the majority of real pop singers lately, and without the need to leave their room to record.What's the appeal of vocoloid? It's all high pitched and annoying to me.
if you're willing to try it out and dive into it, there's a very wide array of vocaloid voicebanks and songs, ranging from classical music to edm to heavy metal. and there's a very wide array of music producers too, each with their own style and touch to it.