Why do they salt butter?
Subsequently, one may also ask, what happens if I use salted butter instead of unsalted?
Technically, yes. You can use salted butter instead of unsalted butter if that's all you've got, especially if you're making something simple like cookies where the chemistry of adding salt in a specific amount and at a certain time won't terribly affect the outcome, unlike bread. The problem is in control.
Keeping this in view, is butter usually salted?
Unsalted Butter. Unsalted butter is all cream, while the salted variety has some salt added, though the amount varies from brand to brand. Because salt is a preservative, salted butter has a longer life in the fridge—typically around five months while unsalted usually is good for about three.
Bakers and chefs usually choose unsalted butter in their recipes because it's easier to manage the salt content in the dish. Most recipes that call for butter—especially baked goods and desserts—are created with unsalted butter. It is the standard in baking and is always implied unless otherwise specified.