Crossovers are like traffic cops, efficient. They make sure the right sound gets to the right component and trim out the rest.
Active Crossover send stronger signals to your amplifiers than a receiver alone can do. A strong, clean signal to your amp means a better sound to your ears.
Passive Crossovers make your woofers and tweeters last longer, and sound better. A crossover works by routing sounds to the speakers meant to play them.