Why do we cream butter and sugar together when baking a quick bread, or any other baked good that uses baking soda or baking powder as leavening?
Baking is both art and science, and this topic is squarely in the science category!
The short answer is that the sugar makes tiny holes in the butter (or other solid fat). When the leavening starts to do its work by giving off carbon dioxide gas (CO2), that CO2 fills the tiny holes, causing them to expand, and giving lift and body to your final product.
Those expanding air pockets also insulate the dough somewhat from the oven heat, so that the butter doesn't melt as quickly and has time to expand even more!
A more complete explanation be found here in one of my favorite food blogs:
Add your eggs slowly after the butter and sugar is "light and fluffy". A lot of recipes will specify to add one egg at a time, which is appropriate for home-based baking. In a larger scale operation we beat the eggs we need just a little bit at first and then slowly add them to the mixture. In any case, beat the mixture until it's pale yellow and "ribbons" (see below). Use a low speed - an electric mixer can substantially raise the temperature of the butter which will cause those tiny air holes to fail. Over-mixing doughs and batters that use soda and/or baking powder for leavening is probably the leading cause of failure of a quick bread or muffin.
For the most part this discussion doesn't apply to yeast raised dough, simply because a yeast raised dough expands through fermentation instead of in the heat of the oven. A slow rise in a yeast raised dough provides ample development of carbon dioxide gas, and the kneading phase provides structure in the development of long protein chains (gluten). I'm disregarding what's known as "oven spring" here, but that's a topic for another post.
Ribboning is when a stream of the sugar / butter / egg mixture dripped back into the mixing bowl forms a "ribbon" for a second or two on top the surface of the mixture. You're doing this to make sure that sugar has fully dissolved, and that you've incorporated in an optimum amount of air (giving you those tiny holes, remember)?
We use extra-fine instead of regular granulated sugar for some added insurance that the sugar completely dissolves.
It's almost a proverb that oil and water don't mix. The fats in butter and egg yolks are both emulsifiers, and in addition to providing a home for those tiny holes, also hold any water in the dough in suspension. So emulsifiers are important for shelf-life as well as structure: Failed emulsions can cause your baked goods to stale by becoming gummy instead of slowly drying out as the water in the product seeps too quickly out of the structure.
Honey deserves an honorable mention here - as well as being an emulsifier in its own right, it adds desirable shelf-stabilization qualities to baked goods. Honey is sweeter than sugar but doesn't have the sharp edges that sugar does to create those tiny holes. We like to substitute honey for half the sugar in a recipe, at half the quantity: For instance, if a recipe calls for a cup of sugar, we'll start testing with a 1/2 cup of sugar and 1/4 cup of honey, while increasing the liquid by 1/4 cup if possible, or if not, reducing the percentage of dry ingredients.
More on ribboning can be found here:
So there you have it, that's why you cream butter and sugar, and then ribbon in the eggs.
You may have noticed that the second post seems to contradict the first post regarding room-temperature eggs and butter. As the first post mentioned, it's a good idea to forget about room temperature eggs if you're using a powerful stand mixer because the temperatures will get too high and your tiny holes will disappear.
This debate continues. There's no doubt that room-temperature ingredients are more effective in trapping air, giving you the lift that you want in your final product. But there's also a risk of temperatures rising too high and your tiny holes booking a flight to San Diego.
Many people think that the advent of powerful stand mixers have made it less necessary to use room temperature ingredients since your they're going to warm anyway while mixing, and that there is a real risk that your emulsion will get too warm when using them, unless you're working with a recipe that uses a lot of eggs for lifting power.
The seasoned baker (pun intended) knows this and will keep a close eye on temperatures at this stage and so is able to use room-temperature ingredients effectively.
The take-away is to mix on low speed and don't let your butter / sugar / egg mixture get too warm - ideally it should 'stay a little cooler than room temperature - between 60 and 65 degrees Fahrenheit. Remember that your sugar (and subsequent flour mix) is probably already going to be at room temperature, so vigilance is your watchword!
You can warm eggs to close to room temperature quickly by placing them into a bowl of warm water for 10 or 15 minutes. You can slice a stick of butter to bring it to close to room temperature faster. Do not attempt to microwave your butter - there's no shortcut here !!!