Arduino code - Why do we need to read the rising edge?
Obviously, they expect you to answer based on the lab or lecture, not based on actual use.

There is no reason you have to interrupt on the Rising Edge instead of the Falling Edge, or the other way round, or both. Or edge interrupt instead of level interrupt.

Your design determines which type of interrupt you need. A signal that is active low will need a falling edge or level low interrupt, while a active high signal will need a rising edge or level high interrupt. 

Sometimes you want to interrupt on both sides of the signal, like from a sensor or button. Someone pressed a button, and you want to do x when it's pressed, and y when it's released.  Or you're timing how long the signal goes from one state to the other. Etc.