As a avid baker, there are generally two reasons people sometimes prefer salted over unsalted butter.
Stays fresh longer. As salt is a natural preservative, salted butter will stay fresh longer either refrigerated or not. Unsalted butter, even when refrigerated, has a tendency to go rancid before people have a chance to use it.
Ambient odors/flavors. Unsalted butters, when left in the fridge, is more apt to pick up ambient flavors, smells, odors, etc. from adjacent foods in the fridge. Salted butter? Less so.
So, there you go.
Plus, people get their diapers all wadded up about being ever so precise in baking. And, yes, generally baking is a precision exercise in measurement (preferably by weight, not volume) but an extra pinch of salt here or there never ruined any baked item – sweet or savory – nor harmed any human being.
So, there you go. Again.