Butter The Difference In Baking With Salted Or Unsalted
You Don't Know How Much Salt Is In the Butter.
The primary reason that recipes don't use salted butter is that you have no idea how much salt is in the butter. This makes it hard to calibrate the rest of your seasoning in the recipe. You can see how much of a range of salt can be in your butter in this article.
If a recipe calls for 1/2 teaspoon of salt and you use salted butter you could end up adding quite a bit more via the butter — sometimes as much as doubling the amount called for.
So unsalted butter is always a better pick when you are cooking so that you can add as much as salt as you want (or don't) in your recipe.