There are two types:
One that goes at the mounts to the bodyshell, moving the beam backwards to put the wheelbase back on a very low car.
I suppose they could work...but why do you need them?
Looks?
Clearance?
I'm really unsure...
The other simply relocates the stub axles upwards on thick plates, whick leaves the beam at the proper angle, but lets the car be very low...all well and good if you don't actually corner your car.
Put it this way, if VW Motorsport braced the rear beam to stop the rear beam flexing and altering camber and toe...what chance has a 10mm plate got of keeping the rear wheels pointing where you want them?
Ans: none.
Waste of money...they are for the retarded that want to run uber low at all costs.