It's a whole thing. The Magicians was just following a trend that genre shows need a musical episode if they've been on long enough. https://tvtropes.org/pmwiki/pmwiki.php/Main/MusicalEpisode
replyDamn I had noticed a lot of series doing it, but I guess it's wider than I thought!
replyThe Magicians overdid it though. It's not a musical episode, iirc it's like 2 seasons
Edit: just to be clear, I still watched those 2 entire seasons, because the story is genuinely that good, plus I was invested. But I really wish they hadn't gone musical, it really messed with the mood of the series.