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. We use this because we have several trains to each station following the same route. If you move the one station and you have like 10 trains it is to boring to change them all over again. It's a must on our games.