"How do you get your cookies so round?" I hear this question a lot. Quite simply, I roll all my cookies—not out of some compulsion for perfection, but for some very good reasons, actually.
Unless you bake a single cookie at a time, you have at least 9 - 12 cookies per sheet. Having the cookies all the same size means they are all equally cooked at the same time, quirky ovens notwithstanding. It is more difficult to determine the volume of a chunk of dough when it is mis-shapen. Rolling the dough creates an easier reference of size. If a recipe calls for flattening with a sugar coated glass or fork, I'll still roll them first to ensure that the pressed cookies are the same size.
Minimal Surface Area
Rolling the dough into equal size balls also gives each cookie the same surface area. A sphere also has the minimum surface area possible for a given mass with a convex shape. Depending on how lumpy your chunk of dough is, the ratio of surface area to mass can vary quite a bit. This will affect the rate of heat absorption by the cookie dough. I like my cookies a little chewy in the middle and the sperical shape delays heat penetration to the center of the dough ball.
Did I mention it also makes the cookies look perfect? Finally, round cookies also pack and travel very well.