The original shims were disintegrated in Mercedes W204.
With living in New England my 12yr old W204 is developing rust on some of the suspension parts, the rear coil springs were in really bad condition. I searched around and found a nice used set of Mercedes coil springs off a California car, had low miles and looked almost new. I bought a set of these shims to replace the original shims which were disintegrated into pieces. I think these are needed to protect the finish on the spring control arms the coil springs sit into, my original spring control arms are rusting where the paint finish is chipped and worn. I noticed once you install the shim it seems to form to that specific control arm, I bought another set of used spring control arms that came with shims installed and they dont appear to fit perfectly if switching around to another control arm, seems even though the control arms are the same part number and look the same there are slight differences maybe from the manufacturing process. I wouldnt re-use a shim if it's going into a different control arm, i'd get new so it forms to that specific control arm.