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Texture, mostly, though a bit of flavor as well.
Okay, so most bread recipes go something like this: mix ingredients, knead dough, let rise, shape, let rise again, bake. There's a lot of stuff in-between, usually just refinements of those steps, but those are the big important ones. The second rise is optional, though, depending on what kind of bread you're making. So, let's go through each.
- Mix Ingredients. This is relatively self-explanitory, but you want to ensure that everything is pretty evenly distributed before you start the real work.
- Knead Dough. This is kind of an optional step, although its results are not optional. The goal with kneading is to form gluten. Gluten is a molecule that's created when you combine two proteins from flour, glutenin and gliadin, with water. The quickest way to form the gluten is to agitate it physically (not emotionally, nobody likes annoyed gluten). If you have a reasonably wet dough, you don't need to do any physical agitation, you can just throw it in the fridge for eight hours or so and gluten will form on its own (this is what's behind the no-knead bread craze). Still, gluten needs to form by one method or another.
- Let Rise. This step is also called "proofing," because if the dough rises, you've proven that the yeast is alive. The main purpose of this step is to convert sugar into alcohol and carbon dioxide by way of the yeast.
- Shape. Before the dough is baked, you will want to give it a form. This form is probably going to determine if you move to step 5 or just do a quick rest and bake. A thin pain à l'anciennes wants to develop completely in the oven, so its shape is kind of a freeform stick. A sandwich bread is a light and puffy thing in a pan, with its size pretty well restrained, so it wants more time to develop.
- Second Rise. If you've rolled your bread into a tube and placed it into a pan, you'll likely want to see the top of the bread crest the pan. "Oven spring"—the rise that the bread gets from the air bubbles in the dough expanding when they hit oven temperatures—will only get you part way up the pan. In order to do a nice, mushroom-shaped slice of bread, you're going to want to give the dough a chance to get pretty close to the top of the pan before you put it in the oven. A second rise achieves this, and it also gives your bread a chance to be fluffier than if you skipped this step. You'll get a bit more flavor development as well, as the yeast is still eating the sugar, but the difference won't be so dramatic compared to the difference between virgin dough and dough after its first rise.
Naturally, there's a little more to it that the above. Shaping the dough moves around all the yeast, so if by chance the sugars from one part of the dough were depleted, the yeast population got moved around to some place with a better food supply. This will keep yeast from dying during the extra development and causing off flavors. With a single rise it doesn't matter, but you don't want the dough just to sit around in its same basic arrangement for the second go around.