They are to squelch the noise, but from having worked on engines with poor service that have these pieces mostly disintegrated and missing and seeing the result I know that they have a little more function than noise suppression only.

The cam lobe for an instant in its rotation has a side that is parallel to shim. The surface tension of the oil will hold the shim and thus the bucket up off the valve. I could be wrong on the exact mechanism here as the valve closing may launch the bucket upwards or maybe it is the inertia of the moving bucket.

When the lobe approaches the bucket going towrds an opening event the parallel side at one point instantly becomes a rapidly accelerating lever and propels the bucket down onto valve top.

The rubber absorber acts as a suction cup as much as anything to hold the bucket to top of valve. It functions when it is in pretty sad shape until it falls into pieces and can no longer give the "dentu-grip" effect.