The sensor bar GND is PWM'd (pulsed on and off) to limit the current through the LEDs (this is called
low-side switching).
Early sensor bars have more LEDs (5 per side) and need the ~7V or so that the motherboard's circuit provides. Later sensor bars only have 3 LEDs per side and can run from 5V and GND directly. To maintain compatibility with all sensor bars, it's best to keep the original circuit and provide 12V to the sensor bar port on the mobo.