Jovius is pretty close to correct. The reality of it is this:
Borg Queen := Plot Device
The writers needed a single “person” to show on screen as representing the Borg. They could have gotten around creating her easily. When Picard was captured by the Borg (“Best of Both Worlds”), it was stated that it was basically for the purpose of having an emissary to humanity. When he was rescued, it makes sense that, perhaps, the Borg would create a “persona” to meet with people.
But no… in “First Contact,” it’s stated that she was there when Picard was assimilated. <sigh> All they needed to do was say that she was created afterwards as an adaptation, and they would have lent a little bit of credibility to their Plot Device.
What I find stupid about the Borg Queen is that–not just in “Unimatrix Zero” but in “Dark Frontier”, “Scorpion”, and “First Contact”–she gives orders.
She gives orders to drones who are telepathically linked to her. Um… why? WHY?
Oh… because the viewer “needs to know” what’s going on. Bring in the Plot Device.
She has also been built up as being plainly evil, whereas the Borg are not evil, but rather mindless–or single-minded, as it were. They expand. They assimilate. They adapt to whatever stands in their way.
The Queen, OTOH, plans, plots, and negotiates. She mindfucks Data. She taunts Janeway. She doesn’t act like a Borg. Why? Because she is a Plot Device.
She is the worst thing that ever happened to the Borg. The only explanation I can put to any of her actions is that somehow the Borg Collective have been influenced by Lore’s plotting and scheming in “Descent.” But… no, in “First Contact” we’re told the Queen existed before that.
Can anyone tell that the main reason I continue to watch Star Trek is so I know what never, ever to do in my pwn science fiction? 
LL