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. If a recipe calls for butter or margarine I always use butter. 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.