A crossover is a type of electronic filter that is used to divide the audio signal into separate frequency bands. It can be used to send low frequencies to the subwoofer and high frequencies to the tweeters, for example. If you are using a system with separate woofers, midranges, and tweeters, then you will typically need to use a crossover to route the appropriate frequencies to each speaker. If you are using a full-range speaker, then a crossover may not be necessary.
In general, it is a good idea to use a crossover if your car audio system includes separate drivers for different frequency ranges. This will allow you to optimize the sound quality of your system by ensuring that each speaker is only reproducing the frequencies that it is designed to handle.
However, it is also important to make sure that your crossover is set up correctly. If the crossover frequency is set too high, the midranges and tweeters may not receive enough power, while if it is set too low, the woofers may be overwhelmed. It is usually a good idea to consult the documentation for your speakers and crossover to determine the optimal settings.