Many Metadata Providers have different offerings for different sets of business partners. In such circumstances it is essential that the web server of a Metadata Provider knows who is calling one of its web services. The same applies for calls to a Release Distributor's web service.
This can be done with or without formal authentication (see Clauses 6.4.4 and 6.4.5, respectively). The benefit of using formal authentication is that it also allows Metadata Providers and Release Distributors to secure their communication.