IDEA is a professional association, like the APA or the ABA. A professional association does a great many things that benefit its members both individually and in aggregate. Members of my family are in several different professional associations, which almost always offer deals on insurance, but I know of none that offer it for free. Actually when I compare the cost of this association with others I know it is relatively inexpensive.
The web site does list specific benefits of membership. One of those benefits is the opportunity to purchase insurance at rates that are less than if it was bought as an individual (just as buying medical insurance is cheaper when bought through a large employer the company tends to reduce rates to an organization that has many members).
Other benefits include discounted opportunities for education, and networking through conferences and online training. There is also the excellent industry publication.
One of the biggest benefits is the professional respect of and connection to others within the field who strive to be the best in the field. We are not just teachers and trainers: we are fitness and wellness professionals.