Some butter is cultured, though it is more difficult to find in the U.S than in other countries across the world. Culturing is a process which is used to enhance the overall flavor of butter as well as to inject secondary flavors which some people can pick up while others simply do not notice them.
The fact is that some people prefer cultured butter over non-cultured butter, usually because the taste is a little bit more interesting and flavorful. A lot of countries in Europe typically culture their butter, though it has not yet caught on in the U.S as much as it has in other places.