As GM said governors keep engines from overspeeding, but they do more than that. Governors on industrial engines keep the engine within its optimal power range and provide a much steadier pull. Think of a tractor that is moving over unsteady ground operating against a constantly changing load. How could you constantly adjust the throttle to keep the engine properly fed? I've run twin engine Cat scrapers, large farm tractors, over the road trucks, etc. There is no way one could constantly manipulate the footfeeds or hand throttles as the governor does.