You can say a lot of things about Joss Whedon, but one thing he's known for is his willingness to experiment with the medium of television. An all-singing episode of Buffy the Vampire Slayer? That just sounds ridiculous on the face of it, but at the same time, what better show to attempt such a thing than a fantasy that has no rules about the kind of events that can happen. In the context of the show, it makes perfect sense that Xander would somehow summon a dancing demon that forces everyone to express their emotions through musical numbers. It's immediately clear that a lot of work went into the musical. While not all of the actors on the show were fantastic singers, they all gave it a shot, and some of them were surprisingly talented. After Buffy went for it and the episode was met with mostly positive feedback, you see other shows copying their model and creating their own musical episodes.