The biggest advantage to being a member of the national IPMSUSA is that they provide the blanket insurance policy for shows put on by local IPMS clubs. Without it, putting on local shows would probably be too cost prohibitive for local clubs if they had to provide the insurance themselves.
Without people joining the national organization, they would not be able to provide this service and the number of shows would decline. While being a member is not a requirement to participate in a show, if no one was a member, there would be no money to fund the insurance policy.
Another thing the organization does in terms of shows is to standardize how things are judged. While judging varies from show to show because of individuals involved, if you know the standards from your club in NJ and decide to go to a show in PA, you're pretty sure of what the standards will be there.