Until the mid-19th century, most cookbooks were simply lists of ingredients with only superficial instructions for the cook. The assumption was that the reader already knew how to cook or bake and could successfully prepare the recipe as long as the correct ingredients were chosen. Modern recipes don't make that assumption. They provide detailed explanations for crucial steps such as separating the wet and dry ingredients in baking.
Baking as Science
Although baking offers tremendous scope for creative artistry, at its core it is a very precise science. For baked goods to reach their full potential, the ingredients must be mixed in the correct ratios. They also need to be combined correctly for ingredients such as butter, sugar, eggs and flour to fulfil their respective roles. This is why wet and dry ingredients are handled separately for baked goods as diverse as delicate cakes and chewy, artisan-style breads.
One of the primary reasons for separating wet and dry ingredients is that they interfere with each other during the mixing stage. If you take flour, sugar, baking powder, salt and spices and drop them in a bowl containing milk and eggs, the ingredients won't be able to mix properly throughout the dough. If you take those same dry ingredients and sift them together before adding them to the wet ingredients, however, they'll be evenly dispersed and create a consistent flavor and texture throughout the finished product. The same holds true for liquid ingredients such as milk, honey and vanilla.
The very simplest muffin and quick bread recipes direct bakers to mix the wet and dry ingredients in separate bowls, then combine them until just mixed. If they're overworked, the muffins will be tough and dry. That's because too much mixing causes the development of chewy gluten strands in the batter. If the wet and dry ingredients were all measured into the same bowl, rather than separated, the mixing required to combine them adequately would produce a heavy, leathery texture.
Emulsification is the process of mixing dissimilar ingredients. Think of a vinaigrette dressing. When you shake it, the oil and vinegar combine temporarily and then separate. Mayonnaise adds an egg, which allows the two to remain permanently mixed. In baking, batters are also emulsions. The dry ingredients help the fat and wet ingredients to stay mixed, producing a batter with a smooth and even texture. This is why, after you've creamed the butter and sugar with the eggs, most recipes dictate that the wet and dry ingredients be added by turns. This technique helps keep the ingredients maintain a good emulsion.