Before somebdy jumps your butt it really isn't a union but it does several things for it's members that Unions also do for there members. As noted lobby for better conditions, but also is a forum for professional expression. It can be a way to float more radical ideas about doing things and getting them talked about without it being a threat to anybodys rice bowl. Probably not a bad idea for a Cadet or JO to belong, and if you get into the Reserves, it can be a bit of a brownie point on evals.
You also get basic information on new systems and equipment before the official poop comes out or at least before it filters to you. It's similar in mission to the Naval Institute, Reserve Officers Associations, etc.