being able to adjust the individual toe of each back wheel is advantageous as well as the camber angle
The std bushes are all goosed by now and are expensive
people prefer both inner and out to be adjustable
some people don't like the inner having to be slotted to make toe adjustments on other makes of beam and you have more accurate finite adjustment of toe with and adjustable rose joint
As we make our own beams we can actually make them any way you would like them