To make our LEDs more efficient, we can put several of them in series. In our 12 volt system, we can safely put 4 of them in series and still have some leeway for voltage droop in our supply. If we put 4 in series, that will use around 8.8 volts.
Of course in the real world, our automotive electrical system is not a steady 12 volts. When the motor is running at a decent RPM and there isn't much load, we would typically see around 13.4 to 13.8 volts in the system. With the motor off a healthy battery should supply around 12.5 volts. While cranking the engine over, or winching with the motor off that can easily drop to 10 volts or less. So we need some "headroom" in our LED circuit to allow for these supply voltage variances.
If we were still using a simple resistor, we might see the LED brightness vary way too much once we put 4 of them in series to get some efficiency. So instead, we employ a current regulator that allows a constant current to flow through our LEDs even while the supply voltage varies under different conditions.