You probably have those that look at it as a hobby, and others who like to volunteer for the benefit of everyone else.
I don't see "public service" and "hobby" as mutually exclusive. Sometimes I deliberately look at the two categories "things I'd like to do now" and "things that would accomplish something useful for someone" -- and pick some activity that's on both lists.
But there are many different motivations, and also many different ways of working.
Sometimes people focus on a particular _topic_ (I like Groaci Nose Flute Orchestras, and I want to make sure everyone on earth can know all there is to know about GNFOs worldwide).
Sometimes people focus on particular _communities_ (This is an interesting group of people to interact with, whatever they're doing, I'll hang around.)
Then sometimes people focus on particular _projects_ (I like to feel that I'm working with a group of people to accomplish something this week -- it doesn't matter whether it's shoveling Augean dung, or eradicating whirled peas: we work hard all week, and on Saturday we'll wash our hands and have a virtual party, then everyone goes off to something else)
And sometimes people have trouble focusing at all, but still work on a series of apparently-random independent tasks (I walk through the directory, starting somewhere at random, looking for sites -- and if I find an interesting site that fits some other category I work there for awhile until ... hey, was that a mockingbird?)
And sometimes people stay in the same mode of operation for a long time, and sometimes people switch modes every few hours or weeks.
The broader the range of motivations and methods the community can harness in support of its goal, the more successful it'll be.