Cross-overs separate the signals between the drivers. THis way the tweeter plays the high frequencies and the woofer plays the low frequencies. If you tried to play low frequencies on a tweeter it would be very easily damaged.
comps.are better that you can take the tweeter and mount it up higher than the woofer to bring the soundstage up.making the music sound more realistic.making it sound like your actually there when the music is played,not just sitting in a box!