Google just came out with their Google Analytics. This is a service to allow statistics about web traffic maintained. It is using the Urchin software that Google bought earlier this year. Urchin created software products that allowed companies to track their web site usage.
Besides Google being a "good" company doing this, I can see other reasons that they would want to do this.
With this tracking information, they can improve page rank. If two sites have the same rank, with the current PageRank system, and they both are using Google Analytics, the site that has more traffic could get a higher PageRank
Also, with this tracking system, Google can know the referrers to each of these pages. Now Google can know the search terms used to get to these pages from other search engines.
With this tracking, Google can determine demographics for each page that has this tracking script. Now they could target the ads displayed to the demographics of the the viewers. Also, this could allow the click through rate to be higher.
[via Google Blog]