What is the purpose of these aluminum shims for the swivel adaptor instead of using the rubber ones? I bought a Monster Tower bracket with post clamp and swivel adaptor for the wakeboard rack and I have 2.5" tubing on my tower.

The purpose metal shim replacing the plastic one is due to the motion of the swivel. The motion can cause the plastic shims to crack, so we require it being replaced to avoid this and avoid damage to your products.