XMI12 — What is the idea of encoding package contents?

Reading through the XMI 1.2 spec, I notice that a peculiar DTD production is
still there: the <9:PackageElementDef> on page 4-10.

In short, this means that if I have say a very simplified UML metamodel,
containing only the "Core" package, containing the single empty class
"ModelElement", I will get the following DTD (fixed elements omitted):

<!ELEMENT Uml:Core (Uml:ModelElement)*>
<!ATTLIST Uml:Core
%XMI.element.att;
%XMI.link.att;
>
<!ELEMENT Uml:ModelElement EMPTY>
<!ATTLIST Uml:ModelElement
%XMI.element.att;
%XMI.link.att;
>

My problem here is this: why would you ever want to use the Uml:Core package
element? Since packages cannot be instantiated, there will never be a
package instance to serialize..

And, interestingly enough, the XML document production part of the
specification never uses the package element either.