what is a crossover used for in car audio work by dividing the frequencies into multiple parts and send them separately to the speakers that can play them. The woofers are used to send the lower frequencies while the tweeters are used to send the higher frequencies. This is why a crossover is an important part used in car audio systems.
The crossover in your car audio system is used to split your audio into separate frequencies for the different speakers in your car. This is necessary because different speakers are better suited for certain frequencies than others. A good example of this would be a subwoofer.