It helps me vent my creativity. If you have multiple interests, being in more than one exposes you to people with varied interests. Just about anything goes in an APA - and often does. Over at APA-LSH, of which this article's author is a member, a mostly friendly debate about evolution has been raging for a couple years. And new parents are proud to run dozens of photos of their babies, and other members are happy to coo over them in their mailing comments.
Members also support each other at times of loss.
And when members get together, the party can run all night or all weekend. Members often organize get-togethers at conventions or their own homes. It's taking it to the next step. You get to indulge yourself in meeting people. A great resource for finding APAs is The New Moon Directory.