I love questions like that, reader Melissa, thank you. Butter is salted for purposes of preservation. Modern refrigeration obviates the need for salt these days, but more than a few people are simply accustomed to the taste.
When butter manufacturers salt butter, they add salt at a proportion of 1 – 2%. That doesn’t sound like a lot, but it goes a long way, especially when you consider how little water there is in butter. Water is the critical element that microbes need to grow in a mass of butter. However by adding that small amount of salt, a brine of about 10-12% salt is created, and that’s an inhospitable environment for bacteria, molds and fungi.