Butter does not have to be refrigerated. Butter can spoil, but it takes longer to spoil than milk. Butter is made from pasteurized milk, which helps to limit bacterial growth.
Many professional chefs prefer to keep butter at room temperature so that the softened butter is ready to use. Butter does not spoil quickly for a number of reasons. It is made mostly of fat and has a low level of water. The salt in butter helps to preserve it as well. The main reason butter spoils is the air, so it is best to keep butter in an air-tight container to preserve it.