Cavity work is performed to try and stop further deterioration in the trunk of a tree. In times past, concrete was used to try to give the tree some structural integrity but this method was wrought with problems and never really found to be sufficient despite the best efforts from professionals. Now ,best practices in the industry have moved toward expandafoam to keep all water out, which has met with a good measure of success.I understand that people do not want to lose a large tree because of a large cavity but many times, good money is spent on a tree to do cavity work only to be taken down a few years later.