It helps keep the body proportioned also. When using a wider angle lens, especially when shooting down on the person like you did. It makes the head and shoulders appear larger and then the legs really tiny.
Of course it works the other way around too, when shooting lower it makes the legs appear longer and the head smaller.
So this comes down to two things. Shooting at a neutral position (the waist) and using a longer lens (I prefer the 85mm).
Heres a great video to watch the explains everything.