For the most part, the www in a domain is pretty pointless. In general it's only there to make you type 4 extra characters to get to the site you want. It can also cause you a lot of pain trying to develop Ajax applications as I once found out.
There I was tying to get some Ajax working on a client site I was busy with only to be given some error about cross domain Ajax being disabled or not allowed. At first I figured I was sending Ajax requests from the development environment to the live site, but after some poking around in the code I saw that Ajax requests were going from http://site.com to http://www.site.com.
This rule will only come into effect if the url starts with the www sub domain, and when its matched it will take all the params (if there are any) and redirect the user to the same place minus the www. You can try it out here, www.dogmatic69.com. Check the url in your browsers navigation bar and you will see it does not have the www in it.
I personally hate getting to a site with a 404 or some other server error, only to find its missing the www in the url.