A dynamometer is a device that applies a load to the vehicle engine, with the load-simulating of the dyno, mimicking the experiences of a vehicle engine when on the road.
Think about it in this way. If you are sitting at a repair shop with your car, with its wheels off and no resistance on the engine - you would not get any accurate readings. The dyno simulates the experience of being on the road, with all the load that is exerted on the engine during that process. It provides accurate diagnostic information about the RPM, torque, air and fuel ratio, boost pressure and other channels. Such data shows what is possible if the engine is tuned to deliver more torque and horsepower.d.