If we just rolled up the cookie dough balls and baked them like that on the cookie sheet, the cookies wouldn't spread much (if at all) and the middle might still be underdone. This is because of the peanut butter in the cookie dough makes it much more dense. By flattening the cookie it can bake evenly, you can use something other than a fork if you want! But I like the nostalgic look of the fork marks.