Very likely the oldest pantomime gag.
And what is it about this age-old genre of theatre the Brits, in particular, love?
Nostalgia. Escapism. Childishness. Colour. High energy. Silliness. Jokes. Ridiculousness.
Directors and actors right now are formulating plots and routines and beginning to rehearse for the pre (and often now post) Christmas runs up and down the UK and abroad too these days for the delectation of expats who just cannot be without their annual belly laugh fix.
Many malign it.
Audiences enjoy it.
Actors thrive on it.
Never mind the ghost, ghoul, spook or monster encouraging the audience to shout the three word refrain as the beast threatens the characters. Maybe the words are just a reminder at the end of the year that so much water has gone under the bridge in the last 12 months and one can look ahead to the coming year.
The past is indeed behind you. It is good to hear it vocalised.
p.s. Me in the Dame outfit.