I like that reason, Noddy. And all of the ones given so far actually.
What I'd add is that for me, I have brain cells that I feel like I need to use or they'll atrophy, and I don't want them to.

So in my stretch of not working (in the paid, outside-the-home sense, though I was working hard as a full-time mom), I volunteered for many of the reasons above but also because I wanted to THINK. I did a lot of administrative/ organizational type stuff, committees, boards, that sort of thing.
I now have a paid job that requires thinking, which I'm happy about, but still am heavily involved in one volunteer organization which stretches me in various directions (did our website from scratch -- though "from scratch" is relatively easy with SiteKreator -- have helped the prez with various delicate situations, am getting ready to write some grants once we get 501 (c) 3 status, etc.) Also really nice for the social aspect, since my paid job is home-based.