I believe they do it because it’s all that’s left of the words of penitential rite that used to be recited in the Tridentine Rite. First the Priest says the Confiteor, then the server/s say: *"*Misereatur tui omnipotens Deus, et dimissis peccatis tuis, perducat te ad vitam aeternam." (May almighty God have mercy upon you, and having forgiven your sins, lead you to life everlasting.)
Then the servers say the Confiteor and the the priest recites that same formula.
After THAT, the Priest says, *“Indulgentiam, (+) absolutionem et remissionem peccatorum nostrorum, tribuat nobis omnipotens et misericors Dominus.” *You old timers and TLM enthusiasts, remind me: the people made the sign of the cross at that point, didn’t they?