Is there any benefits of doing that?; any benefit other than adding more height as he says at here.
His words beginning at about 8:07 :
In the interest of adding a little bit more elevation to this particular head, I took this Manfrotto ball head, put it on top here and then what I'm able to do is actually raise it up... and the reason we do that is so the lens and the camera become what we call plano parallel or level to the birds in the nest... and that's what we want it to do. We want it to be as level as possible.
He's just stacking whatever he has on hand in an attempt to get enough height to get the camera to the position he desires. There's nothing intrinsic about using a ball head on a gimbal head. He's just trying to elevate the camera.