Adjustables allow you to adjust the rear axle left/right to center it (left/right) under the car. This isnt necessary for stock right hide, but is necessary when the ride hieght is changed (ex: lowering the car).
That being said....
I have stock right height, but I do have adjustable Spohn rod/poly PHB and LCAs. They allow for the adjustment of the rear axle in all horizontal directions (left, right, forward back, diagonally). My rear was a bit off with the stock parts and is all straight now with the adjustables (adjusted by the mechanic during an alignment...you have to ask for it and may need to convince them to do it).
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So the whole purpose is to strengthen the rear suspension or to make a more solid handling feel and to center the axle if the vehicle is lowered.
JEGS sells a bracket for the panhard bar and states that it is necessary when lowering a car. I do not see anyone else selling a version of this bracket http://www.jegs.com/webapp/wcs/store...1319&langId=-1 so is it really necessary or is the adjustable panhard bar all you need?