Salted vs Unsalted

Your favorite baking recipe calls for unsalted butter PLUS salt, but why can't you just use salted butter instead?

Seems counterintuitive, I know, but using unsalted butter + salt, rather than salted butter allows you to be more precise in the amount of salt you are adding to the recipe.  There is no industry standard for the amount of salt a stick of salted butter contains, so you could be adding anywhere from 1/4 to 1/2 teaspoon of salt per stick, and that's a pretty big margin.

Mmm...tasty!