What does a hair light do, and do you need it?

I finally got some radio triggers for my flashes. They are a cheap Chinese job but they work fine - and even better, I can use them with any camera because they are just "dumb" flashes with no TTL or any of that new-fangled mess!

I've always known that I should be using a "hair light" in a simple 3-light setup, but I've always tried to "KISS" - keep it simple stupid. Well, I tried out that 3-light shot on Meagan last night, in preparation for a quick shoot today, and it really added a new dimension to the portrait. It removes the subject from the background very nicely.

Here is the photo. Setup was one light 45° left, a little higher than the subject. A fill light 90° right and twice as far away was fill. Finally, the hair light was slightly behind and to the left of the subject over her head, at 1/4 the power than the main light. I'm always looking for ways to improve, so feel free to critique. Oh yeah, taken with a Nikon D800E and 85mm f/1.4 lens at f/9, and slightly edited in Photoshop: