Most cocktails degrade when multiplied for a crowd. Punch does not, because it was designed for the bowl from the beginning and solves the problems of scale within the recipe itself.
The structure is a ratio, not a serving
The traditional formula sets out proportions of spirit, citrus, sugar, water and spice, which can be multiplied without any component becoming awkward.
Because the water is written into the recipe, punch does not depend on ice-melt to reach its final strength.
That single difference is what allows it to be made hours ahead without becoming either strong or watery.
Tea does the work that bitters do elsewhere
Traditional punch recipes use tea as the water component, contributing tannin and aromatic complexity along with dilution.
The tannin gives the drink a defined finish and stops a large volume of sweet citrus liquid becoming cloying.
Strength and steeping time matter, since over-brewed tea makes the whole bowl astringent and cannot be corrected once mixed.
Oleo saccharum extracts oil rather than juice
Peels are buried in sugar for several hours, and the sugar draws the oil out of the peel and dissolves into it.
The resulting syrup carries far more citrus aroma than juice does, which is what gives a well-made punch its aromatic lift.
It also solves a practical problem, since the peels are a by-product of the juicing the recipe already requires.
A large ice block controls the dilution
A single large block has far less surface area per unit of mass than the same weight of cubes, so it melts slowly.
This keeps the punch cold across an evening while adding only a modest amount of water to a recipe that already contains its dilution.
Cubes in the same bowl would water the punch down within the first hour, which is the most common way a good recipe is ruined at the last step.
Resting improves it
A punch mixed and left cold for a few hours integrates, with the citrus, spice and spirit reading as one drink rather than as components.
This is the opposite of a shaken cocktail, which is at its best in the first minute and declines from there.
Because it improves with time rather than degrading, punch is the only major format where making the drink in advance is the correct method rather than a compromise.