Eggs

Egg yolk adds flavor, enrichment and color--egg white adds neither but body either when whipped or used without whipping.

If you have no acid ingredient in the batter, the recipe calls for baking powder. But any acid ingredient (yogurt, sour cream, buttermilk) upsets the acidity balance (pH)  and to bring the batter back to neutral, the recipe calls for baking soda (an alkali ingredient).

George (Author of What Recipes Don't Tell You)

In fact the name for "egg white" in German is "protein."  This protein can be put to good use to get more gluten stretch from low gluten flours.  One does have to be careful to avoid long warm ferments when including egg whites in a recipe.   The yolk adds fats to the dough and color.

Mini

Baking soda will, when combined with an acid like buttermilk, vinegar, or tartaric acid, combine to release carbon dioxide gas, which makes the bread rise and not be a brick.

Baking powder is a mix of chemicals designed to react and release carbon dioxide when combined with water or heated.  A classic recipe is baking soda and dry tartaric acid (also known as cream of tartar).

If you have a recipe which has acidic components, like milk, buttermilk, vinegars, wine, beer, etc, then you don't need the added acid that's in baking powder, and you can use straight baking soda.