I like this fellow’s summary of Wikipedia’s appeal:

They were drawn because for a work of reference Wikipedia seemed unusually humble. It asked for help, and when it did, it used a particularly affecting word: “stub.” At the bottom of a short article about something, it would say, “This article about X is a stub. You can help Wikipedia by expanding it.” And you’d think: That poor sad stub: I will help. Not right now, because I’m writing a book, but someday, yes, I will try to help.