Chilling the cookie dough allows the butter to solidify again which helps to prevent the cookies from spreading out too quickly while they bake. Chilling the dough also allows the liquids in the cookie dough a chance to hydrate the flour blend which will help to develop the flavors of the cookie and promote an even bake. If you skip this step, you will probably end up with very flat, thin, sad pancake-like cookies instead of thick, chewy, and tasty cookies.