The creaming method utilizes the plasticity of butter to create holes in baked goods for leavening to occur. The procedure is to mix sugar and fat together first, then add the eggs, and finally to alternate the addition of dry and wet ingredients (beginning with dry ingredient additions and ending with wet ingredients). Beating the sugar into the butter give creaming method baked goods (like cookies and cakes) their characteristic uniform hole texture and appearance. The sugar punches tiny holes throughout the butter. These holes become the basis for the rise of the product via carbon dioxide produced from chemical leavening.
The amount of creaming influences the final product. The more you cream butter the more cake like the final product will become. Light and fluffy creamed butter is great for cakes, but may not be desired in cookies. For most cookie products, it is best to cream just until the butter and sugar are incorporated into a paste. Any more than that and you will have cakey cookies.