I've been waiting for one of our resident experts to comment on this, but since no one has - here goes: I think it was used as a noise/vibration dampener. You got me to thinking about this with your question, so I called some of the local brake shops and they all said the same thing. My original thought was that they were used as a heat-sink to dissapate brake heat, or possibly used to keep the drum from warping under extreme heat conditions, but was told that wouldn't be necessary on rear brake drums because they only get about 25% of the brake load. Hope that helps!
