The dough is mixed the day before, then chilled. The AOP butter has to stay firm without cracking: too warm and it escapes, too cold and it tears the dough.
Twenty-four hours later the layers have relaxed. In the oven, the water in the butter turns to steam and lifts the whole structure. That's what makes the honeycomb you hear crackle.
We could go faster. The croissant would be paler, softer, and you wouldn't come back.