Why does bread dough need to prove twice?

Because the two rises do different jobs. The first develops flavour and structure; the second creates the final texture of the loaf.

The first prove, sometimes called bulk fermentation, is the long one. Yeast consumes sugars and produces carbon dioxide, alcohol and a range of organic acids. Those acids and alcohols are where most of bread's flavour comes from — a dough given a long first rise tastes noticeably more complex than one rushed. Meanwhile the gluten network continues to organise itself, becoming stronger and more extensible without any further kneading.

Knocking back — deflating the dough afterwards — is not undoing the work. It redistributes the yeast into fresh food supply, evens out the temperature, and breaks up the large irregular bubbles that formed during the first rise.

The second prove, after shaping, produces the bubble structure you actually eat. Because the bubbles now form in a dough with a well-developed gluten network and are broken up more finely, the crumb comes out even rather than full of large holes. It is also what lets the loaf hold the shape you gave it.

The second prove is much shorter, typically 30 to 60 minutes.

Skipping the second rise gives a dense loaf that tears unpredictably in the oven. Skipping the first gives bread that rises adequately but tastes bland.

Many artisan methods extend the first rise dramatically — a cold overnight prove in the fridge is common — precisely because flavour development is the slow part and cold slows yeast without stopping enzyme activity.

The test for a finished second prove is a gentle poke: the dough should spring back slowly and incompletely.