Unsalted butter has a more consistent percent of water in it. If you’re using salted butter your yield (the number of servings) will be lower and will vary from one brand of butter to the next. This will be more evident with the larger the batch you make. Also, I have known people who say they just use salted butter and omit the salt from the recipe. I don’t believe that is a wise choice because you’d have to do a fair amount of math to figure out how much salt is in the butter and then compare that to what you need for your recipe.