Why should a diode be used with a protective resistor?

Question

In a physics experiment, I used a diode connected in series with a protective resistor and measured the current and potential difference across it. Does anyone know why it's necessary to include this protective resistor when using a diode?

Answer 1

In the forward direction, a diode has little resistance (less than 1/2 ohm). It can potentially short out the power supply or battery. The extra resistance puts a cap on the max load on the power supply as good measure.