The creaming method is commonly used when making cakes, this method gives the cake a light and airy crumb. The goal of the creaming method is to aerate the batter. When you properly cream, your butter will become very light in color due to the inclusion of millions of tiny air bubbles. By adding the sugar and butter together you will increase the volume of the fat in your batter. This is because the sugar acts as an abrasive which cuts holes into the fat, which then seal themselves over, creating tiny bubbles. This way leaveners like baking powder can produce their gas which fills up the little pockets like balloons. If you did not cream the fat and sugar together the baking powder would not be able to create the same light texture in your baked good.
I think the easiest way to cream is in the bowl of an electric stand mixer. You would need to cream by hand for up to a half and hour to get a similar consistency to the 5 minutes in a stand mixer. Place your room temperature fat (shortening or butter) into the bowl and using the paddle attachment mix the butter on low to break it apart. Then add your sugar and turn your mixer up to 50% (5 or 6 on a Kitchenaid stand mixer). Make sure to scrape down the bowl especially the bottom every couple of minutes. I usually cream anywhere from 4-6 minutes. You are looking for a very light color in the butter and the volume to increase by about double. After the butter and sugar have been properly creamed it is time to add your eggs one at time and let them incorporate. In a separate bowl sift together all your dry ingredients. These will be added alternately with whatever liquid you are adding (milk, buttermilk, sour cream, yogurt). I start with a third of the flour and let it mix in, then do the same with half of the milk, and continue like that till everything is combined. Make sure not to mix too much at this point you don’t want the gluten to develop in the flour because you will end up with a very tough cake. The creaming method is vital to the texture of your cakes and even cookies so don’t omit the step or else your baked goods could suffer.