Optional, but recommended, the power relay harness protects the vehicle's factory wires. It bypasses the original harness and connects the HID kit directly into your car's battery.
Xenon headlights require more power than stock halogen bulbs. We recommend you use the power relay harness as the factory wiring might get damaged in the long run.
The power relay harness also prevents the headlights from flickering as it provides necessary amount of electricity to the HID lights.