This page discusses the aspects of the global architecture of registries which influence the decision at to whether it is necessary to install a registry.
The IVOA registries for a distributed network on the internet. Registries periodically replicate data between themselves in a process known as "harvesting". So registries contain effectively two classes of information;
The resources that a registry 'owns' are separated from the other resources by belonging to a particular set of authority identifiers, which provide a namespace.
The decision about whether it is necessary to install a registry depends on whether you have permission to add resources at a particular registry - i.e. if the curator of a registry is prepared to allow you to enter resources in their registry then it might not be necessary to install a registry yourself. However, even if you can obtain this permission it can still be advantageous to install a local registry.
Once the registry has been installed, if it is desired that other registries should possibly harvest from it, then the details of the newly installed registry should be entered at the "registry of registries" at http://rofr.ivoa.net/