Using butter is just a personal preference. Shortening has different properties than butter. When a recipe calls for butter or margarine, it is to add flavor as well as texture to the recipe. Shortening does not add flavor. In just the opposite situation and a recipe calls for shortening- use shortening.
Shortening is whipped to give loftiness to recipes.
Butter and margarine can be substitied for each other because measurement and weight are equal in volume. Simply insert your own values between the double-quotes.