It sounds silly, but there’s a reason.
A quarter-pound stick of typical salted butter may contain 1½ to 3 grams, or up to half a teaspoon, of salt.
Different brands and regional products may contain very different amounts. When you’re following a carefully formulated recipe, especially one that uses a lot of butter, you can’t afford to play Russian roulette with something as important as salt.
That’s why serious, high-quality recipes will specify unsalted or “sweet” butter and leave the salt for a separate seasoning step.
Many chefs prefer unsalted butter also because it is often of higher quality. Salt is added partially for its preservative effect, and butter that is used promptly, as in a restaurant kitchen, doesn’t need it.
Also, in unsalted butter any “off”-flavors, such as incipient rancidity, are more readily detected.