You should know about this important difference between salted and unsalted butter.
So you have a recipe that calls for unsalted butter, but then tells you to add salt in the very next step! What gives?!
The main reason for this is control. When you use unsalted butter, and then add your own measurement of salt separately, you have complete control over the amount of salt in your recipe.
Conversely, you have much less control when using salted butter, since unfortunately different brands of butter contain different amounts of salt.
Unsalted butter will also stay fresher longer, and we all know how important it is in baking to always use the freshest butter!