Same reason as multiple mid-woofers and woofers - to achieve higher sound pressures.

Rebulx, I was watching you moving your ear from the top to the bottom tweeter and then back to top again. It looked like you were doing the funky chicken. Don't do that because you'll melt your cochlea. Or put your back out, dude.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated