... is because it ties together thematically a lot of related content and, sets the context of "community" in very neat terms.
John Poelstra blogged about this some (well, actually, quite a while) back, at http://johnpoelstra.com/2008/02/23/miserable/ by drawing from the 'miserable job' factors. Every community with a purpose also needs to have ways to ensure that the 3 factors of : anonymity, irrelevance and immeasurability are avoided.
I missed a mention about this in the article, but is the source to http://rcos.cs.rpi.edu/projects/ available online ?