They are, I suspect, a symbol of freedom to a girl who was bound by an uncaring system, they're living breathing creatures that lift her up higher than she everthought she could be, and carry her faster than she dreamed possible. They also need her, and she needs them, and neither betrays that trust. Of course they're very important, to her thinking, every hoofbeat they make is a challenge to what's wrong in the world, and the flash of their manes a defiant banner against it that will not yield and is there for her.