Charlie,
Dunno why myself - suppose it began when there weren't many flying wimmin around and they needed solidarity not to look like individual freak shows? Less relevant now, but they have men in their numbers too these days.
My personal interest is really just wanting to meet as many people who share my interests as possible and maybe get advice or whatever from time to time - they seem to be a fairly unique organisation and I don't think there's one without 'women' in its name that does exactly the same thing.
Haven't investigated AOPA very thoroughly, admittedly, but it seems a lot more geared towards owners (which I won't be able to afford to be for a long time!) and private pilots with no further ambitions (which I'm not). Also, it seems larger and less personal and therefore of limited use. And other more intimate societies are for specialist interests that I don't have (yet).
However, if I remember rightly, this impression is based mainly on a Pilot or Flyer (can't remember which) article some time back and may be hideously inaccurate, so any info on this also is welcome.
(Edit - yes, and Circuit Basher 's reason was the other thing, but I wasn't going to mention that!)