If you ever have to annotate one tree using a completely separate tool, you need to do something to ensure the names you choose are different from the ones used

If you ever have to annotate one tree using a completely separate tool, you need to do something to ensure the names you choose are different from the ones used by whatever produces the tree. You can choose odd names, but there's no guarantee. Namespaces can provide that guarantee, via uniqueness of URIs, ultimately relying ICANN.