Sound is divided into frequencies. The low frequencies a person can hear is around 20-30 hertz, 30-500 hz (low to upper bass), 500-2000Hz (human speech), 2000-8000Hz (higher human speech, pianos, etc), 8,000-16,000Hz (sound of the letter "s", bells, etc.) There is no single speaker that can reproduce the whole frequencies from 20 to 16-20,000Hz. That is why we use several speakers to handle sound. Subwoofers for lows, midrange speakers for mids, and tweeters for highs.
When the amplifier sends out a full range signal ex: 50Hz - 16,000Hz and you have separates to accept those signals (a tweeter and a midrange speaker or woofer) you need to divide the frequencies in such a way that the lows will go to the midrange/woofers speakers and the highs will go to the tweeters. Another name of the crossover network is the dividing network. The highs will not go to your big speaker and the lows will not go to your tweeters and just distort your sound and eventually destroy your speakers. So, in effect you will have a high quality sound. Sound set-up is an art. There are several ways to install depending on a lot of factors. It starts from the type of music you prefer, to the type of vehicle you have, to the amount of space you are willing to give-up, to the equipment preferred, etc., etc.