Way back in the dark ages, houses had a main panel and sub panels. The main panel was grounded but the sub panels were not. The neutral and the ground were common (connected together) at the main panel. Extending both the neutral and ground to the final use is kind of a belt and suspenders approach adopted for more recent electrical codes (if there is an issue with the neutral wire you still have the protection of the ground wire). (Electrical gurus can explain this better.) In older houses without grounding systems I often (for my own use), replace two prong receptacles with three prong receptacles and tie the ground to the neutral similar to time2roll's suggestion.
