So far as I'm aware, it's simply tracking stats of those who have the Alexa Toolbar installed - which is invariably webmasters, looking to boost their Alexa ranking.
I'd use it myself for that reason, actually, but the toolbar has a bug that causes problems with vbulletin, from my experience - sometimes when posting you suddenly end up with dozens of new
windows opening up simultaneously, and every time you close one, a new one opens. You simply have to wait for it to finish.
