DARKSPILVER said - AFAIK by joining up with the GAA, congregations avoid having to pay for commercial insurances (which would cost more?)
The org's alleged insurance scheme? When my previous KH was seriously damaged, two RBC hitmen were sent from the Branch to have a meeting with the Elders. They tried (unsuccessfully) to get the congregation to pay for certain aspects of the repair work, but were successful in requiring that the congregation members carry out many hours of clean up work and painting, before the RBC were sent in. And we waited months for the hall to be done.
So the answer is this - the org's insurance may cost the congregations more.