It also serves as an anti-magnetic measure. Here are the 2 pieces that were removed, when converting my current-gen Speedy Pro to a Thomas Priek custom caseback. The top one is the 'dust cover', and the bottom the interior of the actual caseback.
I was sad to lose the protection against magnetism as an outcome of the conversion. However, the protruding ring and hippocampus on the OEM caseback was cutting up my wrist because of how sharp it is. Since replacing the OEM caseback with the Thomas Priek sapphire one, my Speedmaster is now my most comfortable watch!