The CacheRegistration serves to uniquely identify all entities that operate against the data cache. It is usually associated with a validated client session.
When a client sends a LoginRequest, the LoginResponse will contain the originatorID associated with this registration. Using this ID, the client is able to determine whether or not events it's receiving were caused by its actions or those of other clients, which can be useful in generating warnings etc. For example, if you're in the middle of editting an object and you receive an event to indicate that it has been modified by someone else, you can present a warning.
You can have a look at the org.dmd.mvw.client.mvwcomms.extended.CommsController class for a concrete example of how the originatorID is used.