After I asked my twitter followers what step in cooking they did without knowing why, Elizabeth told me:

@thefoodgeek don't know why I add flour slowly or eggs one at a time.

Hi, Elizabeth,

This is a step that is popular in the creaming method of cake preparation. In the creaming method, you start with butter and mix, or cream, the sugar into the butter. Then you add your eggs, one at a time, and then iu add your liquids and dry ingredients in 4 to 6 alternating batches.

Sounds complicated, right? It's not really all that bad, though, and it's all pretty important. Let's go through one at a time, and roundabouts step two we should have an answer to your main question.

Creaming the sugar into the butter is to create little pockets of air in the butter, which is one of the things that causes the cake to rise in the oven (as I wrote in my article on Ingredient Temperatures). 

The next part, of the most importance to us right now, is the adding of the eggs. The goal here is to get the eggs completely incorporated into the butter. One of the things that doesn't necessarily occur to us is "what does it mean for the egg to be incorporated into the butter?" I mean, we can pretty much recognize that the butter/sugar/egg mixture is thinner than the butter/sugar mixture, and you can't see any egg any more, but where did the egg actually go?

It turns out that the egg gets emulsified into the butter. Butter is a combination of water and fat, which traditionally don't go well together. An emulsification is a combination of oil and fat that actually works. They are the opposites that do attract. Well, they attract if you put something in between them that draws them both together. The "something" is an emulsifier, and egg yolks contain a really good emulsifier called lecithin. You can read more about emulsification in The Buttercream Nemesis.

So, butter is an emulsion, and eggs contain water and fat of their own, as well as great emulsifier. This is all useful, but when you're making a cake, you want the eggs to be completely gone, not to have some combination of scrambled eggs and butter. By adding eggs one at a time, you help to ensure that the eggs mix with the butter, rather than just mixing with each other. 

Once the eggs are appropriately emulsified, you have the wet ingredients and the dry ingredients to incorporate. If you were lazy and didn't love whomever you were giving the cake to, you could just dump all that in together, mix it until it's consistent, and go about your business. But if you were doing that, you would just run down to the corner grocery store and buy a cake that tastes of food coloring and cardboard. But no, you want better than that, so you will not dump everything in all at once.

Instead, you will add half of the flour. Potentially a third of the flour depending, but half is usually good. You'll mix until it's just barely combined. Then you'll add half of the liquid, mix until combined, then the rest of the flour, then the rest of the liquid. Why does love (and, by extension, cake mixing) take so many steps?

Gluten.

Yes, I am always, always, going on about gluten. That's because it's simultaneously beautiful and terrible. Gluten, for those who haven't been reading all of my articles up until this point, is what you get when you combine two proteins in flour, glutenin and gliadin, with water. It give baked goods body, bite, and structure.

In a cake, you really don't want a lot of body. You want lightness, and joy, and maybe some frosting. So you limit the amount of gluten that is formed. So when you add the flour to the butter emulsion, the flour gets coated with fat, or with fat and water molecules that are emulsified together. Because fat either hates water, as is its wont, or the water is so tied up in the emulsion with fat that it couldn't possibly spend enough time and energy on glutenin and gliadin to make gluten, minimal gluten is formed.

Once all that flour is safely tucked away in the fat, you can add the water, knowing that it won't bind with the glutenin and gliadin because, as we said, it's surrounded by fat.

Now, we still have some flour left which, in an ideal world, we would have completely mixed into the butter. Unfortunately, in order to make the cake into something that has any structure at all instead of a puddle of sweet, melted butter, we have to add a bunch of flour. So much flour that, if we tried to add it all, wouldn't spread out evenly in the butter emulsion, but would instead form clumps. And I can assure you that a clump of flour does not a tasty cake make.

Because of the threat of clumpiness, we risk a little gluten formation by adding the half of the water, then the rest of the flour, then the last of the water. It's traditional, and a perfectly good way to make a cake, which will end up both delicious and moist.

The great thing is that now, aside from knowing why you add eggs in one at a time, you also know the secret to making any cake with the creaming method. If you understand the why well enough, you won't even have to worry about most of the steps of the recipe. You'll glance at it and say, "Hey, this is the creaming method!" You'll cream the sugar into the butter, add the eggs, then the liquids and dry ingredients. Recipe steps will be unnecessary, and people will marvel at your skill with cakes.