In some cases, such as the M80s, it improves power handling--ie, more drivers (ie midranges, tweeters, woofers) can play their given frequencies louder with less potential for damage.
Multiple woofers (or mid-woofers, if you prefer) exist for similar reasons, but the target there is moving more air--the more air you move, the louder it is. Using multiple woofers is sometimes considered better than using single larger woofers because of packaging and stiffness of the material (which translates to more accurate reproduction).
On the center channel, I believe the additional tweeters aid with dispersion, but I'm not certain there.
Also, Bose only pretends to be high end. They're really not very good speakers at all. Stick with manufacturers such as Axiom, Paradigm, Aperion, Wilson, Polk, Klipsch, etc.
