Speaker crossovers

I want to dip my toe in making a pair of DIY open baffle speakers using one 15 inch coaxial driver per speaker. Can anyone explain (at the 4th grade level  :)  why I need crossovers for coaxial drivers? Seems to me if I need crossovers, I might as well use separate drivers. I ask because I'd like to use a couple of these: 


To complicate things the specs recommend XO-4 crossovers which I've found only to be manufactured in Italy. I thought the whole advantage to using coaxial drivers was so one wouldn't have to bother with crossovers. Guess I'm wrong. Please explain why and how one would hook up the crossovers. Also, what would happen if I didn't use crossovers (decrease the sound quality of the drivers or amp? Destroy or damage one or both)? Are there any high quality 15 inch coaxial drivers with "built-in" crossovers? I need "simple"  :(
Mew, it is not that complicated. The speaker you chose is a speaker mounted within another speaker, two separate speakers mounted coaxially. If you send bass to the high frequency driver you will likely damage it. You can design your own crossover. There are tools on line to do this but I would just get the recommended crossover. 
Check out Parts Express and see what they have for drivers.
the crossover is there for protection to prevent bass going into the tweeter and highs going into the woofer. This is very basic knowledge
When it gets to where guys who know less than kenjit are building them you can be pretty sure the open baffle fad has just about run its course.
Post removed