In our church the sacristans do way more than just set up for Mass. They:
*]Collect the money from the votive offering and put out new candles every morning
*]Put out bulletins and song sheets on Saturday evening
*]Clean and replace water in the holy water fonts and make sure Father blesses the water when more is needed.
*]Order hosts, altar wine.charcoal for the thurible, incense, candles, and all other consumables.
*]Launder altar linens, corporals, purificators, hand towels, altar server vestments, funeral pall, etc.
*]Lock the nave of the Church before Masses and unlock after Masses
*]Polish the sacred vessels, clean and polish thiribles, aspergillum, whatever the holy water thing the aspergillum goes in is called, etc.
*]Change batteries in the microphones
*]Complain when altar servers check under the pall to see if the sacristan forgot a host, or that its to hot in the Church, or too cold.
*]Reprimand RICA directors when children don't genuflect properly
*]Drink coffee twenty minutes before Mass
*]Boss priests around
*]Say, "That's not the way we used to do it" constantly. :D
And a whole bunch of other stuff that I can't remember right now. The idea is to take all those cares away from the priest so that the priest can prepare spiritually for the Mass and then pray the Mass, without having to worry if an altar candle is going go out during the preface because no one replaced it or something silly like that.
The priests and our pastor provide oversight, correct and advise, but they greatly admire, respect and rely on their sacristans. Our lead sacristan and the other senior sacristans are very good at what they do.