The answer to the question of "how to get more women into tech" is simply "get more women into tech" — if women see other women in the tech industry, they'll be more likely to perceive it as a viable career path, and their numbers will go up.
But when you have the present situation — where women are still a minority — it's hard to start that positive feedback loop. Women don't see other women in tech, so they're less likely to see it as a potential career path, and their numbers stagnate.
So the idea behind an event like this is to help break the perception that "women don't do tech." If you have a concentrated event with lots of women programmers, women are more likely to show up just because they see at as an event they are welcomed at.
Having an all-womens' programming event also helps alleviate a lot of the pressures that prevent women from showing up at coed events. Ask a female programmer you know about how it felt showing up at their university's computer club, or a hackathon, when they were still relatively inexperienced: they'll probably attest to feeling as if they needed to "prove" their geek credentials, or they'll talk about being afraid of looking like some incompetent girl (as opposed to simply incompetent). All-female tech events help women build up confidence so that they feel "equipped" when/if they choose to tackle the larger tech community.