Butter is without a doubt important in baking for a number of reasons, including the fact that it adds a unique flavor and texture to certain things like cookies and cakes that you will definitely notice if you do not use it. Although there are other butter substitutes, it is the best kind of fat to use in baking because of the fact that it is natural, contains no carbs, and has no trans-fat whatsoever.
Shortening and margarine are two common substitutes for butter, though they typically do not have the same exact taste and the finished product can come out tasting or looking just a little bit different. Margarine does however have no saturated fat unlike butter which has lots of it. So those who are trying to stay as healthy as possible when baking might want to think about using margarine instead of butter.