Butter comes two ways: salted and unsalted. Salt is added to butter for flavor and as a preservative so it will have a longer shelf life. Salt, however, can sometimes overpower the sweet flavor of the butter and can also mask odors. Additionally, the amount of salt added to salted butter varies from manufacturer to manufacturer, so it's hard to know how much extra salt you're adding to a recipe. Using unsalted butter allows the chef to control the amount of salt in a recipe.