Do you contribute to a scientific society (as a member or otherwise), and why? Many scientists don't bother maintaining memberships, thinking the only benefit is getting a journal to which their institution already subscribes. Most people don't even think about what scientific societies do. The best answers I have heard are "manage the journal", "organize meetings", and/ or "give out awards." Many societies do some or all of these three things, but I think most academics are unaware of the backdrop, and the importance of the financial linkage to the journals and conferences. Scientific societies are sometimes funded largely by profits from their journals and meetings, though some funds also come from membership dues, donations, and grants (the relative proportions vary greatly by society-- e.g., some take a yearly loss on their meetings). The revenue funds are then used for maintenance of those same activities as well as distributing a variety of awards (most often for students but sometimes for workshops), outreach activities, and overhead/ staff. Some societies also engage in activism, promoting the merits of their discipline to the public or politicians, including drafting statements about controversial topics (e.g., stem cell research, evolution education) and arguing why scientific research is a good public investment (see also this article, that's not by a scientist).
These societies are, and operate like, non-profit organizations. However, the revenue source is a little different-- rather than relying exclusively on donations, the yearly conferences and journals provide potentially larger and more consistent funds to maintain these activities.
Now, let's think for a minute-- what would happen if we changed the model to one resembling most non-profit organizations? What if society journals exactly "broke-even" to the publishers, making them cheaper in which to publish and subscribe but eliminating the revenue to societies? What if we then asked PI's to donate to their societies to fund the grants, outreach, and activism efforts? There's a fundamental problem-- "donations" cannot be billed to grants, so we'd have to donate out-of-pocket. This problem is apparent in why people often refuse to pay memberships to get reduced publication charges, even when the reduction is greater than the cost of membership-- publication charges come "from the grant" and memberships come "from checkbooks."
So, societies with an associated journal get funds from journal publication and subscription charges (and sometimes from conference registrations), and use those funds in a "charitable" way to meet the goals of the group, including distributing extensive student research or achievement awards, maintaining communication among scientists via conferences, and providing outreach to schools and the broader public. Yes, there's some overhead too, like funding the travel of the scientific officers to meetings, but these officers rarely receive any compensation beyond "expense reimbursement" for what ends up being days of work each summer. There are typically minimal (and often overstretched) staff, too.
This should be food for thought in choice of publication venue, personal society memberships, personal journal subscriptions, recommendations to your university about journals in which to subscribe, and choice of conferences. Memberships in societies (e.g., Sigma Xi) really do directly give back to your community, in addition to getting you a journal. Subscribing to and publishing in SOCIETY journals (e.g., Evolution, Genetics, Journal of Heredity) brings money back to your students, colleagues, and community, whereas publishing in NON-SOCIETY journals often fails to do so. Similarly, attending SOCIETY meetings potentially gives flexible funds back to an organized group with similar goals, whereas attending other meetings only does so indirectly, if at all.
Food for thought. I'll note that I'm not unbiased, though, as I have served and currently serve in many societies myself.