Not all recipes call for unsalted butter. However, when a certain recipe calls for one, it just means that the recipe accounts for no other source of salt. Unsalted butter is widely used in baking because unsalted butter can help you control the correct amount of salt that you will add to your dish.
If you use salted butter for baking, you will guess how much you should adjust the salt in your recipe and you would be blindsided on how salty your dish will become. Although salted butter can be used in some cases especially when unsalted butter is not available in your local supermarket. However, if you are going to use this, make sure you cut the instructed amount of salt in half.