Not 100% sure on this question either. My take on it is...........
With 0 volts on the signal input all leds will be on, as you increase the voltage on the signal input each led will go off 1 by 1 as the voltage gets higher (0 to 5 volt)
A use for it might be to indicate a battery voltage (although dont know of a 5 volt battery) and as the battery got flatter more leds came on.
Another use might be a sensor indicator as long as the sensor varied the voltage between 0 and 5 volts.
Also worth noting it would work in 1 volt steps, at 1 volt all leds on, 2 volt 3 leds, 3 volt 2 leds, 4 volt 1 led, 5 volt no leds.