The purpose of using unsalted butter in recipes and then adding salt
separately is to allow you complete control over the amount of salt that
goes into the recipe. Regular butter has salt in it, but you cannot
measure the amount exactly -- each brand varies to some degree.
BTW, I usually buy only unsalted butter in my household since I do so much
baking. When I want to serve butter on the table (e.g., when I'm having
friends over for brunch or dinner) I serve tiny rectangles of it,
"embellished" with kosher salt sprinkled on top and a tiny sprig of
parsley. It has the multiple purpose of adding salt for the flavor but
also makes a beautiful addition to the table setting.
On another note but relating to butter topics, I've been eye-ing the
"butter bell" that I've seen in the King Arthur catalog. Has anyone used
these things -- do they really work well? I was thinking this might be
just the thing when I make a big spread of baked breads and take them to
the office where we don't have refrigeration options -- this might allow me
to keep the butter out all day with the array of breads and other
condiments so that it doesn't go rancid.
Hope that helps,
Joan