subdomain - Should I default my website to www.foo or not?

Question

Notice how the default domain for stackoverflow is http://stackoverflow.com and if you try to goto http://www.stackoverflow.com it bounces you to http://stackoverflow.com ?

What is the reason for this? Not the tech reason (as in the http code, etc) but why would the site owners want to do this?

I know it's purely aesthetic and I always have host-headers for both www and not, but is there a reason to bounce a user to a single domain, subheaded or not?

Update 1

Not having a subdomain is called a bare domain. Thanks peeps! never knew it had a term :) Update 2

Thanks for the answers so far - please note I understand that www.domain.com can point to domain.com. This is not a question about if i should offer both or either/or, it's asking why some sites default to a baredomain instead of www subdomains, or vice-versa. Cheers.

Jeff Atwood actually HAS explained why he's gone for bare domains here and here. (Nod to Jonas Pegerfalk for the post :) )

Jeff's post (and others in this thread) also talk about the problems of a bare domain with cookies and static images. Basically, if you have cookies on in a bare domain, then all subdomains are forced also. The solution is to purchase another domain, as posted by the Yahoo Perf Team here.

Answer 1

Jeff Atwood has written a great article about the The Great Dub-Dub-Dub Debate. There is also a blog entry in the Stackoverflow blog on why and how Stackoverflow has dropped the www prefix.