Why is it recipes call for unsalted butter but then have you add in salt.

so you get precisely the right amount of salt to achieve the desired result.

amount of salt in butter can vary.

The amount of salt in salted butter is way more than the small amount of salt you add. Plus salted butter has a higher water content than unsalted butter, and unsalted butter is sweeter than salted butter. All these things effect a recipe so they are really not interchangeable.

Salt is a preservative, and salted butter can last two to three months longer in the refrigerator than unsalted butter. So this actually means that salted butter is often much less fresh than unsalted, and sometimes has been made from cream that is less fresh as well. Overall, it's best to buy and use only unsalted butter for cooking and baking, especially since you can't even reliably determine how much salt is in any given stick. A dear pastry chef friend explained an estimated 3/4 teaspoon can be in a stick of salted butter, but this varies depending on brand and place of origin.