I have in the past been a member of the ASA, AIAA, and AES. These are mainly academic organizations; they make most of their operating expenses by selling journal subscriptions to university and other libraries. (You think dues are expensive? Look at the library rates for the journal!!) They put on conferences where technical papers are presented; these papers feed the Journal. Academics and corporations get some prestige from publishing, but no money - in fact there are usually fees for reprints of your own article. Often universities and corporations will pay for attendance at these conferences, especially if you are presenting a paper.
If you are not writing papers for the conferences and journals, then your benefits of membership are a low price on journal subscriptions and discounted fees to attend the conferences. The other advantage, with some societies, is there may be local chapters which have meetings, typically monthly or quarterly. These can be quite worthwhile; there is usually a presentation and always a chance to network and "geek out" with others that share your interests. Many of these local chapters are open to people who are not, or are not yet, members of the parent society. I think it's a good way to try it out, it's how I started.
Incidentally, as an undergrad I participated in some oceanographic cruises on Coast Guard ships. The ETs were very knowledgeable and helpful, and saved our bacon more than once. It was a privilege -and an education - to work with them.
