This section is from the book "The American Woman's Cook Book", by Ruth Berolzheimer. Also available from Amazon: The Domestic Arts Edition of the American Woman's Cook Book.
Eggs Bind Fats and Liquids Together - Fats and liquids mixed together tend to separate very quickly. When egg is added to this mixture, it is possible, under right conditions, to secure a very intimate mixing of the fat and liquid. The best known household illustration of this is the combination of oil, vinegar and egg in mayonnaise dressing, which produces mixture that will keep for a long time. In the case of French dressing, the oil and acid can often be held together for an hour or longer if a small amount of egg-white is added.
Eggs Help to Combine Ingredients in Batter and Dough - Although many batter and dough mixtures, such as cakes, muffins, pancakes, and breads, may be and often are made without egg, the use of egg materially improves them. Egg brings about a very intimate mixing of fat and liquid not only with each other but also with the other ingredients present. This gives the product fineness of grain, particularly in mixtures containing fat, and increases its lightness of texture. Cake, fancy yeast breads, such as zwieback, brioche, rusks and fancy rolls, and quick breads, such as delicate muffins, owe a part of their delicacy of texture to the presence of eggs in the mixture.
Egg Increases Power of Batter or Dough to Hold Fat - By causing a more intimate mixing of fat with other ingredients, the egg in a batter and dough mixture will permit the addition of more fat. If a cake is so rich that it has a tendency to fall, the addition of another egg may cure the difficulty. If it is not rich enough, yet falls when more fat is added, putting in another egg permits the use of more fat. If richer muffins are desired, the same rule holds good; eggs as well as fat may need to be added if the product is to retain its lightness. In fancy yeast breads such as zwieback, brioche, rusks and fancy rolls, the large amount of fat present does not reduce the lightness of the mixture, in part at least because of the effects of the egg present.
Egg Increases Power of Batter or Dough to Hold Liquid - Egg causes the liquid to be distributed in smaller particles throughout a batter and dough mixture. This makes it possible for the mixture to hold more liquid, without interfering with its lightness, than it could hold if the eggs were absent. Therefore, a bread or cake dough made with egg can be made softer than one in which egg is not used. This adds to the delicacy of the product. The popover is the most interesting illustration of a batter that is very light in spite of the large amount of liquid present.