Why is it that girls get hair under there arms?

I've heard three theories.

1.  It wicks away sweat for cooling. 

2.  (related)  As the sweat goes so does bacteria, keeping the skin healthier.

3.  It traps pheromones.  Primitive man liked it like that.  😉

I have no idea if any of that's true.

I thought everybody knew this. It's where their stuffing comes out.