adjustable rear control arms-what do they do? what are they for?

 

The only reasin that you would ever need adjustable LCAs is if your rear isn't perpendicular to the centerline of the car you could shorten or lengthen one to square it up. Or if you want to run large daimeter tires for dragracing you can move the axle forward or rearward to center it in the wheelwells. Other than for those two purposes they are a total waste of money.

 

They allow an alignment shop to do a thrust angle alignment, which is making the rear tires point straight compared to the fronts.