Lights have another use, besides allowing others to see you & reducing your range [by a tiny amount].
In certain circuits, like this one for example:
BMS bypassed with loopkey and discharging with lights/accessories circuit
The lights also act as a mine canary — if they shut down while you are riding, then you have a serious battery problem and need to pull over and shutdown immediately. But this is without cutting your brakes or shutting down your drivetrain.