Why use salted butter in a recipe? Does using sweet butter {which I only ever buy} and then adding salt significantly change the results?

The reason for using unsalted butter is so that you can control the amount of salt---going by taste---in a recipe. Important if you are making sauces.

Here are a couple of pickle threads with great answers about salted vs. unsalted butter in baking.
http://www.food52.com/foodpickle/4266-baking-with-salted-butter
http://www.food52.com/foodpickle/3627-which-is-best-to-bake-with-salted-or-unsalted-butter