According to Nobel Laureate, Robert William Fogel, we must have a sense of purpose in order to enjoy a life feeling fulfilled (see Citizendium article, The Fourth Great Awakening and the Future of Egalitarianism).
Fogel ranks sense of purpose first among a long list of non-material resources required for leading a rich life in terms of quality. To have a sense of purpose means that you have “something that one sets before himself as an object to be attained : an end or aim to be kept in view in any plan, measure, exertion, or operation”, and that you know that you have it (see ‘sense’ and ‘purpose’ in Webster’s Third New International Dictionary, Unabridged. Merriam-Webster, 2002. http://unabridged.merriam-webster.com).
Because many of Citizendium’s volunteer authors write regularly and with enthusiasm, voluntarily and without byline-type credit#, I can only surmise that their doing so fulfills some inner need they have. It seems to me, after over five years with Citizendium, that their need falls into the broad category of pedagogy, sharing their knowledge with others, giving enrichment to the lives of others, a type of charity.
What they seem to get for themselves: the enrichment that comes with having a sense of purpose.
If all that seems a bit idealist, but not certainly so, try your own hand at sharing your knowledge through writing in Citizendium. You could also try doing it in some other free online wiki encyclopedia, like Wikipedia. Or try two or three and see which environment accords best with your sense of purpose.
# Citizendium does not include bylines with article titles, but every entry+save you make will appear on the ‘History’ page of the article with your registered name.