Butter adds flavor and richness to the dough, but has a lower melting point than shortening, which can cause cookies to spread more during baking and result in thinner, crisper cookies. Shortening, on the other hand, has a higher melting point, which helps cookies hold their shape and results in thicker, chewier cookies. It also has a neutral flavor, which allows other flavors in the cookie, such as vanilla and chocolate in this instance, to really shine through. In general, if you prefer a richer flavor and don't mind thinner, crisper cookies, using butter in your cookie recipe is a good choice. If you prefer a chewier texture like I do, shortening is a better option. I do have other cookie recipes that use butter, such as these Slice-and-Bake Cookies, but these chocolate chip cookies are specifically formulated to be made with shortening and I recommend following the recipe.