Why do I need PIDs?

In the emerging cyberinfrastructure we are creating more and more references between resources, resource fragments and services. The creation of these references is very costly and often is essential for the interpretation of a resource. Therefore we need proper mechanisms to ensure that these references survive despite all the changes that happen in repositories for example. It is known that URLs are not appropriate - they are not persistent even when we believe that they are proper URIs. Therefore special PIDs come into place which identify an object and which are maintained by reliable institutions.