Socialtext developer Chris Dent has a long and well considered post on social software and why he does wiki.
I think there are three primary audiences for wikis: the individual who hopes to use the wiki as an outboard brain or memory; the nascent group that hopes to discover and solidify the community that lies as potential in their loose connections; and the existing community that hopes to support a shared goal or perform some action.
…With both blogs and email, content tends to be relatively static. Typos may be corrected in a blog entry and email threads may carry on forever but there is little in the way of refinement of the content. This is where wikis step in: they are good tools for summarizing, annotating and connecting information. These are the actions of a knowledge enhancement system.
He also writes Ross Mayfield’s Weblog which focuses on markets, technology and musings.