NetDB III controls access to classes and subclasses with object access security and access to specific instances of objects with object ownership security. Node Owner is designed to give a NetDB user update access to a specific instance of a Node that she normally wouldn't have.
For example, say NetDB user trixie doesn't have accesses to objects of type router (a node subtype). Now suppose we want to allow trixie update access to router shovebits, but no other routers. There are two ways to do this:
Node Owner may never be used. It was added to the NetDB data model with the thinking "gee that's possible and it may be useful someday". It continues to be supported in the data and the code supporting the data for the same reason, but no other code has been written to take advantage of it. That's because implementing it is nontrivial, and, at this time, there's not a lot of demand for what it provides.