Usually, computer hard drive activity is indicated with a single flashing LED when there is write or read operation performed. [Harvey Wilson] wanted something different than just a flashing LED and came out with an indicator. He replaced a single LED with 10 LEDs mounted in a circular shape.
After digging some info from the Internet, he found the HDD signal nature and built the 4017 counter based circuit which accepts each incoming LED signal and then increments a counter. All 10 LEDs are connected to counter output pins, so each event lights makes the feeling of running lights.