Unsalted butter + salt ???
Salted butter also contains more water ( which can make it less stable when creaming) than unsalted and often contains annatto( a colorant).
The salt also gives the butter a longer shelf life for better or worse(depending whether you are storing it or if the store is).
Different companies add varying amounts of salt so you really do not know how much salt you are adding which is a problem in baking especially since you can not adjust the seasoning after the dish is finished. in general you reduce the amount of salt in the recipe by .25tsp for each .5 cup of butter.
I like unsalted butter with salt. The reason like calmossimo is that you can control the level of salt when using unsalted butter. So I almost always buy unsalted butter. I can use it for unsalted butter recipe and salted butter recipe.
And other reasons is that not all salted butters have the same level of salt. So it is just easier for recipe books to request unsalted butter.
it's so you can control the amount of salt in your own cooking. there are different amounts of salt in different brands of salted butter so you won't know how much salt you're adding to your food.
however, i'll say that i have successfully subbed salted butter to a number of recipes and just made a guess of how much added salt to reduce by (usually i'll guesstimate between 1/4-1/2 tsp) and things have turned out fine. maybe my palette just isn't refined enough to be able to tell.