Of what I've read about swinging, it's not something that is decided upon before the couple married, but it became something they decided to after.
A couple I know got into swinging because she wanted a girlfriend. They didn't want some young skank or hysterical/bipolar woman with issues, she wanted someone reasonably mature, self supporting and personally honest, who didn't "want" something (eg trying to get to her husband/assets) and that unlike poly wouldn't be dealing with her kids as any part of the relationship. Somewhere safe, with an available woman who was relaxed and open or single (and without disease).
Her husband wanted similar, although he was happy with their sex life, had no interest in other guys at all, but was mildly curious about seeing two (or more) women together.
Swinging (according to the brochure) would fit those ideas.
What she didn't count on was, many of the people weren't in good shape (her and her husband were early 40's, not athletic but "tidy"), that she didn't really get "a girlfriend" as there were much fewer women actively participating and those that did tended to have their own little "harems" of male advocates, with the occasional attached woman. And so instead of getting "a girlfriend for fun and games" she initially ended up with a bunch of "part time husbands" (which is apparently lots more work, and not so much of the fun [I get the feeling that one is expected to 'entertain' rather than just be an equal, in such 'relations']. She did in the end get some parttime girlfriends and her husband found that he had a mild fetish for cuckolding, and she was surprised at the strength of her negative reaction at seeing him with another female (even one that was just the three of them). But they're still happily together, although they don't attend much anymore, from what I hear.
Also, fyi, it seems not a lot of poly-folk get into the swinging scene. which is mildly surprising, that therefore swinging tends to be for monogamous couples (or younger women looking for sugar daddies/mommies, from what I hear...)
PS apparently it is amusing when you get socially (or professionally) introduced to someone for the "first" time, that you've only met before at the swinging group
. Since you have to act as if you've never met them before, but you know them (or their partner) rather well (and sometimes some of their fetishes.....)