But nobody ever writes anything using Cb.
So having two young girls, Frozen is of course "the thing".
The song Let it Go is written in Ab major, with the intro in F minor. Now five flats is pretty exotic to start. But then you get to the end of the chorus and it's suddenly a bit dissonant, and that dissonance then continues into the bridge. If you look at the notes out of context, it looks like a B major chord. But no: the song has modulated down a double dominant to Gb, and that is a Cb major (IV) chord.
As far as I can tell, the entire point of the Cb major chords earlier than the bridge is to anticipate the modulation down,
just for the purpose of a double dominant modulation halfway through the bridge.