... kind of sounds like overkill, but there's reason behind the confusion!
Imagine that you're trying to find some data. In the USGIN scheme, in order to find data you search through CSW services, which are catalogues full of ISO1939 metadata documents. Each of these documents points you to a dataset or service that is somewhere, hopefully online, for you to take a look at. These CSW services are like the card catalogues of the digital era.