Other than being able to access the global before the singleton is instantiated I don't know.
*EDIT*
I suppose a possible reason to use globals rather than fields could be that a module that uses the same singleton class can access them without knowing the name of the instance (but it's not really that useful).
*EDIT*
I suppose a possible reason to use globals rather than fields could be that a module that uses the same singleton class can access them without knowing the name of the instance (but it's not really that useful).