>Why do we need more girl scientists and geeks?
You are only considering a symptom of the problem. It isn't just a question of more women as geeks or scientists. It's how women are treated in society. Things like getting catcalled while walking to class. Being called a slut for wearing a skirt, a prude for wearing a dress, manly for dressing in slacks, or sexy rather than professional.
No, we don't need more women as scientists, or doing math, or building things, or designing video games, or writing books, or plays or tv scripts. And we don't need women collecting garbage, serving in combat roles in the military or elected to government.
But the sum total of a person's experiences add up.
When someone in a video games says "You got raped, bitch." Or a little girl feels shamed into playing with bratz dolls rather than legos. Or while walking down the street a stranger demands she smile because she's so pretty, ignoring she might have her own feelings. Or she plays a video game where the female characters all have impossibly giant breasts spilling out of impractical armor. Or when watching block buster movies where no two main character women (if there even are) talk to each other about something other than a man. Or every time she steps out side someone helpfully reminds her that she could be raped at any moment; she better accept a man's offer to chaperon her. Oh, and why hasn't she married a nice man yet and had some children? She's not getting any younger you know. It all shapes who a woman is.
So no, we don't need more women working as scientists. But if you work to build and maintain a culture where women are guilted, shamed, and literally forced into stereotypically feminine roles because that's the way it is and you're content with that then you certainly help to guarantee she won't be curing cancer, building a better engine, developing a faster computer, or a creating a realistic lead woman character in the next blockbuster game or movie.