Why do some old tractors have to shake with iron bars when they start?

All the engines you see, whether diesel or gasoline, are in a stopped state and in a working state. It needs an external force to turn the crankshaft of the engine before it can be started. This is a necessary condition. The tractor you saw was too backward to start without electricity. Hand cranking is the most effective way to start it. Of course, there are other ways to start, only if the former doesn't work properly.