More to advertise it. A WS is made available by publishing its WSDL, but only people who know the URL of that WSDL will be available to make use of that WS. But if the service is in a UDDI registry, then people can go out looking for services and be able to find it. But note that UDDI was a short-lived fad, and isn't much in use any more these days. It was envisaged that UDDI registries would enable a marketplace of services, but that never happened.
Ping & DNS - updated with new look and Ping home screen widget
I get the impression that UDDI is alive and well but only inside private corporate systems as a means of coordinating services. Think SOA (Service Oriented Architecture) for example. Competition in this area includes ebXML and a whole bunch of other approaches to coordination.