When you choose the values in a passive crossover, you pick them based on the impedance of the driver. But most drivers have an impedance that varies with frequency so your crossover won't work properly unless you "correct" for that. With active crossover's it's not necessary, since the crossover is before the amps and never sees the driver.
In a car, I don't know if it's really necessary though. Alot of the times you have such a horrible in-car response that your crossover isn't anywhere near accurate anyways.