Nutritionists and doctors aren't usually great promoters of butter's virtues. It's high in saturated fats, and that's where the conversation ends. Bakers have a rather different perspective, favoring butter for the richness and flavor it gives to their baked goods. Many commercial recipes specify unsalted butter, and as long as you're eating it in moderation -- and you should be, anyway -- there's no reason not to use it in your baking. It's an easy substitution, even when it's not called for.
Fats in Baking
Every fat has its own characteristics, but they do the same things in baked goods. Their most important role is softening the crumb, giving cakes and soft rolls a delicate texture that's noticeably different from the crusty, chewy texture of a good bread. They're also important to the chemistry of your baked goods, binding up the sweeteners and flavoring ingredients with the flour. Finally -- and this is where butter shines -- they can add richness and flavor to your baking.
Unsalted vs. Salted Butter
A certain amount of salt is crucial in baking, even in sweet things. In yeast doughs it helps regulate the yeast's activity, and in anything baked it helps bring up the full flavor of the other ingredients. If you normally use salted butter for baking, substituting unsalted butter will reduce the total amount of salt in the recipe. Sometimes, that doesn't change the flavor noticeably. In that case, you can just use unsalted butter and cheerfully enjoy the lower amount of sodium in your diet. If you find the flavor becomes flat without the extra salt, add a quarter-teaspoon of salt for every stick of unsalted butter.
Shortening and Oil
Substituting unsalted butter for other fats such as shortening and oil is both simpler and more complicated. One one hand, you won't need to adjust the salt in any recipe calling for oil or shortening. On the other hand, butter isn't a pure fat, but usually contains about 15 percent water. That's a difference of approximately 1 tablespoon of liquid per cup of butter. In small recipes it won't matter, but if you're baking a monstrous batch you might need to reduce the liquids accordingly, and add a little more butter to keep the fat content where it should be. If you're substituting for oil, melt the butter so the batter will have the correct consistency.
The Other Way
If you ever need to go the other direction and substitute a different fat in place of unsalted butter, you simply make the same adjustments in reverse. To use salted butter, reduce the salt in your recipe by 1/4 teaspoon per stick of butter. If you're using shortening in place of the butter, reduce the amount by 1 tablespoon per cup and increase your liquids by the same amount. If you want to simulate the richness and flavor of the butter, use buttermilk for your added liquid.