There are several reasons why you may need to have a valve, whether you’re on an oilfield site or not.
In Canada, if you fall under CSA B620 or B621 standards then Transport Canada will require you to have a positive air valve installed on your vehicle.
Some companies make it mandatory for their whole fleet to have a positive air valve installed in a vehicle, often due to a previous incident.
If the valve shuts down the engine when it reaches the high RPM shutoff limit then it is possible for it to cause damages to your engine, however it is unlikely. Those damages are minimal compared to an engine runaway.
The test RPM functions are designed to make sure that the system is reading RPM. This function will close the valve at a safe limit that will not damage the engine.