It's useful to copy orders if you have vehicles that all have the same route. For example you have 10 airplanes and have to set them to 10 airfields. It's boring to look for the air fields over and over again.

To copy the orders of a vehicle you simple click on Go To in the orders screen (Note: The vehicle must have no orders on that moment) and click on the vehicle where you want to copy the orders from.

On #openttdcoop we don't use this kind of coping orders but we use Shared Orders. This is a MUST for our train networks.