I'm probably going to regret asking this...   why is there a separate division for women?

Because In chess the strongest players tend to be males, and there are more males then females in general.

Therories why vary a lot, from sociological impact (less women persue chess, so there are less top women players), to biological ones (men are more inclined towards activities utilizing localized processing skills)

Since this is going to come up, the only scientific study pertaining to the differances between men and women comes from the royal society.  They subscribe to the viewpoint that since there are more men playing chess, there are naturally going to be more players at extreme levels of skill (Grandmasters).  They conclude that men and women are equally good when it comes to chess, you can agree with this or disagree if you like.  Personally I don't show any opinion publicly because I simply don't know enough.

the biological differences between men and women (many of which are entirely the result of estrogen, testosterone, and progesterone) do not affect one's reasoning skills or memory.  however, since testosterone acts as an accelerant, perhaps men (in general) are more driven to competetive gaming? 

I'm sure that socialization aspects influence the number of men and women in chess as well, but that is based upon a much larger set of variables.

(insert opinion here)

Personally I think it is rather redundant, obviously there may be physiological
differences between men and women that will give one an advantage in chess. But once you start on that path you may be able to say the same for other factors race for example.  Also rich people mayhave an advantage over poor people.  The list goes on.   You could end up with many "divisions".  I believe pit everyone together, maybe "women" won't be able to compete at the top, but should that matter?

the current world women's chess champ is Alexandra Kosteniuk.  Her web site is  www.kosteniuk.com    she is soooooooo  cool!!!