Built into the theme and gimmick of the episode, too, since the conceit was that people in musicals can sing things they would never say. Buffy's reveal that she had been in heaven, not hell, and that part of her regrets coming back was something epic, just an incredible move to drop that on the audience in that form.
I also don't doubt SNW will make some attempt for the episode to leave some significance in the overall story. This whackadoo crossover episode was important for Uhura and as I saw noted on Trekyards of all places, the actual payoff of "Ad Astra, Per Aspera" for Una.
I may have started the "waste of an episode" idea, so I should clarify that I didn't mean to equate that the way Axaday did with not advancing the plot. I meant that if someone doesn't like musicals, then a musical episode is an all-consuming episode gimmick, and they're likely not to enjoy anything about the episode. In fact, I'd consider "Ad Astra, Per Aspera" a "waste of an episode" from my perspective, because it was a trial episode played straight and I find them painfully boring. I felt I'd got little or nothing for my time and just had to wait another week for an episode, so it's a wasted opportunity for me when it might have been an episode I would have enjoyed. But there's no denying it's a significant episode within an ongoing plot arc, it ended quite an interesting one.
Plot's not the only way for a thing to have value within a show. Like, you know, "Far Beyond the Stars", which advances themes instead, and affects what the events mean instead of what happened next. And SNW is a deliberately episodic series, too!
Edit: I do think the odds of a gimmick episode also being "skippable" from a plot sense are somewhat higher by convention, but call that correlation rather than causation.
