A Global ID is an app wide URI that uniquely identifies a model
instance.  This is helpful when you need a single identifier to
reference different classes of objects.  One example is job
scheduling. We need to reference a model object rather than serialize
the object itself. We can pass a Global ID that can be used to locate
the model when it's time to perform the job. The job scheduler doesn't
need to know the details of model naming and IDs, just that it has a
global identifier that references a model.  Another example is a
drop-down list of options, consisting of both Users and
Groups. Normally we'd need to come up with our own ad hoc scheme to
reference them. With Global IDs, we have a universal identifier that
works for objects of both classes.