Chilling a dough does four separate things and only one of them is temperature

The fat has to be solid, and that is the obvious part

Butter is workable across a narrow band, firm enough to stay in discrete pieces and soft enough to be rolled rather than shattered, and warm hands and a warm room push it out of that band quickly. Chilling puts it back. In a shortcrust, pieces of solid fat coating the flour are what interrupt the gluten and produce the short texture; in a laminated dough they are the layers themselves. Once the butter has melted into the flour, neither is recoverable by cooling it down again.

Cold fat has a second effect in the oven that has nothing to do with mixing. Solid butter takes longer to melt, so the layers or pockets it forms survive further into the bake, and by the time they do melt the surrounding structure has begun to set around the space they occupied. Warm butter melts before anything has set and simply leaks, which is why a warm laminated dough bakes flat and greasy.

This part of the instruction is well understood and it is the reason people give when asked. It is also the least interesting of the four, because it explains only why the dough should be cold and not why it should sit.

Water needs time to distribute itself

Flour absorbs water slowly, and a dough mixed just to the point of coming together has water sitting unevenly through it, with some regions wet and some barely damp. Resting lets moisture migrate until the whole mass is evenly hydrated, which is why a rested dough rolls out without cracking at the edges while a fresh one crumbles and needs patching.

The effect is more pronounced the drier the dough. A wet pastry dough evens out in half an hour; a stiff, low-liquid one benefits from several. It is also why the amount of water a recipe specifies always looks slightly wrong at the moment of mixing, and why adding more at that stage is such a common mistake. A dough that feels dry in the bowl is often perfectly hydrated by the following morning.

Gluten relaxes only if you let it

Mixing and rolling stretch the protein network and put it under tension, and a tensioned dough contracts when released. That is the mechanism behind a tart shell that shrinks down the sides during baking, a rolled sheet that pulls back as fast as you push it out, and biscuits that come out of the oven smaller than they were cut.

Given time, the bonds within the network rearrange under that tension and the stress dissipates, a process bakers call relaxation and materials scientists would recognise by the same name. Cold slows the chemistry slightly but the time is what matters here rather than the temperature. This is why the rest between rolling and baking is at least as important as the rest after mixing, and why the second one is the one people skip.

Enzymes and flavour work on their own schedule

Flour arrives with active enzymes in it, and given hours they break a little starch down into simpler sugars and a little protein into amino acids. That combination is exactly what the Maillard reaction needs, so a dough rested overnight browns more readily and tastes more complex than the same dough baked immediately. The difference is most obvious in enriched and yeasted doughs and is genuinely detectable in a plain cookie dough as well.

The commonly repeated claim that a chocolate chip cookie dough improves markedly over a day or two in the fridge rests on this, alongside the hydration and the firmer fat that reduces spread. How much of the improvement belongs to each factor isn’t something a home kitchen can separate cleanly, and the honest position is that all three are contributing.

Where chilling is the wrong instruction

Not everything benefits, and a blanket rule doesn’t survive contact with the repertoire. Choux paste is piped warm and suffers from being held cold, since its structure depends on gelatinised starch that stiffens as it cools. A creamed cake batter left to chill loses the aeration it was built for, because the butter firms around the bubbles and the batter becomes harder to spread evenly. Yeasted doughs are a case of it depending: a cold retard develops flavour beautifully and slows the rise to the point where timing becomes a matter of judgement rather than a clock.

And there is a limit at the other end. A pastry dough chilled hard becomes brittle rather than plastic, and rolling it straight from a cold fridge cracks the butter into shards instead of sheets. Ten or fifteen minutes at room temperature before rolling is standard practice for exactly that reason, which makes the full instruction less tidy than the recipes let on: chill it, then warm it slightly, then work quickly.