Why Tag?

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

Empirical evidence suggests that tagging is most useful when public and shared.  But why, exactly?  Caterina Fake, in a panel at Syndicate, noted that people on Flickr get to "ride free" on top of compulsive categorizers.  I think this is certainly part of it, and maybe tagging is good occupational therapy too, but I have a gut feel 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 the key point -- this is a natural behavior, and a social one.  (He'll talk to himself, but it's really second best -- he wants to share his view of the world with other people!)  And of course it's accompanied 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 social interactions.  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 the same way that talking to yourself might be useful (yes, sometimes, but not a primary use case).  More interesting is social-but-private where you share with a limited number of people; this is more difficult to do well than either totally private or totally public; is it valuable?  How?  When?