Rolling cookies in sugar before baking helps to create a crackly top, which is why crinkle cookies are almost always sugar-coated. Sugar is hygroscopic, which means that it attracts and soaks up water. By adding sugar to the outside of the cookie, water in the dough is drawn out as it bakes, so the tops of the cookies dry and crack while the center remains soft. But don't worry - even with these crinkles on top, these cookies are chewy throughout!