Thanks for the prompt reply. The case you mention is the only one I could think of, but having a global work item area seems more to overcome the problem of not being able to query across multiple project areas? Is it not more usual to have related components and work items in the same project area? In that case is there any benefit in being able to link change sets to work items outside the project area?

It looks like I misinterpreted your original question ... a change-set belongs to a component, not a project area, and a component doesn't really belong to a project area (although it usually gets its read-access control from a particular project area).  It is very common to have multiple project areas all make changes in a single set of components.    On the other hand, a change-set does belong to a repository, and so I was interpreting your original question as being "choosing a work item in a project area in another repository".   So probably a better answer to your actual question would have been:
When you have multiple project areas making changes to a given component, it is common for a change-set to be associated with a work item in the project area that is making that change.

ok, thanks for the clarification, but regarding my original question, when associating a change set with a work item you can choose a work item from a different project area, when would that be useful?

also your comment 'multiple project areas all make changes in a single set of components', if I understand correctly having read-access to another project area's component allows you to make changes to the component in your project area but you cannot deliver those changes back to the owning project area, for that to happen you would need to be a member of the owning project area - is that correct? If so, would that be a situation where it would be meaningful to associate a change set with another project area i.e. your workspace flows with a stream in one project area but you associate the change set with a work item in the other project area that owns the component?