What does a Crossover do? Why buy a Crossover?
The job of a crossover is to split your audio signal into different bands based on the frequencies and then these bands can be routed to the speakers that are best suited for those bands. In other words it sends the bass signals to the subwoofers and the high-frequency signals to the tweeters. By using a crossover your subwoofers will no longer waste energy trying to process the entire acoustic band but only the frequencies that it can handle. This makes for added clarity in your car audio system and also gives the owner more flexibility when it comes to fine tuning the system.