A PD controller can be used in a real-time on a robot where a goal value can be both measured and controlled. A PD control is robust, which means that it can overcome unexpected behavior.
For example, a PD controller could be used in the cruise control on a car. The driver decides the goal speed and the controller in the car could see how fast the car is going, since cars have a speedometer. Under ideal conditions, all it would take to maintain speed is to keep the engine throttled at a predetermined constant amount. However, many unforseable factors can change how much gas is needed to maintain speed. For example, the quality of the gas, engine temperature, road incline, and wind speed. A controller needs to be able to overcome these unpredictable or difficult to measure sources of error to maintain a given speed and accelerate or decellerate as needed.