Why Tag?

One of the questions that keeps coming up in discussions about taggingis whether private tagging is useful and if so, how?  Is publictagging really the important application to keep in mind and if so,why?  By private tagging, I mean someone applying tags but notsharing them with anyone -- so they're useful for personal organizationbut not for sharing with others.

Empirical evidence suggests that tagging is most useful when public andshared.  But why, exactly?  Caterina Fake, in a panel atSyndicate, noted that people on Flickr get to "ride free" on top ofcompulsive categorizers.  I think this is certainly part of it,and maybe tagging is good occupational therapy too, but I have a gutfeel there's more to the story.

My fifteen month old son is an inveterate tagger.  His tag cloud looks something like this at the moment (somewhat elided):

airplane água ana bird book bulldozer bus bye choo-choo-train dada dog down mama phone tractor truck up wow

...which I know because he tags things repeatedly and excitedly,especially when someone else is around.  And I think this is thekey point -- this is a natural behavior, and a social one.  (He'lltalk to himself, but it's really second best -- he wants to share hisview of the world with other people!)  And of course it'saccompanied by pointing -- the original hyperlink.

That's as far as I've gotten.  Fortunately, Rashmi Sinha, in A social analysis of tagging,does a great job of analyzing exactly how tagging facilitates socialinteractions.  Go read it.  Also, read her earlier cognitive analysis of tagging as well.  Both great forays into the "whys" of public tagging.

I think this all suggests that private tagging might be useful in thesame way that talking to yourself might be useful (yes, sometimes, butnot a primary use case).  More interesting is social-but-privatewhere you share with a limited number of people; this is more difficultto do well than either totally private or totally public; is itvaluable?  How?  When?