Imagine a carbohydrate as a solid brick wall. It would be impossible for the bacteria to bite this food, it's simply too large. The airborne wild yeast creates enzymes needed to break down this wall into manageable bitesize pieces for the bacteria to eat. This interaction releases carbon dioxide which is trapped within tiny bubbles and results in the dough expanding, or rising, producing the leavening of the bread. This synergistic interaction also creates lactic acid, acetic acid and ethanol alcohol.
Lactic acid contributes to better digestibility of the bread product and gives it a unique flavour.
Acetic acid acts a flavour agent and natural preservative. It lowers the pH level of the bread product retarding mold growth. Ethanol alcohol creates that wonderful smell of freshly baked bread that is so enticing. All sourdough baking uses this soupy mixture to produce a variety of different bread products that can't be made in any other way. The longer fermentation time of sourdough baking breaks the protein down into amino acids making the bread more readily digestible.