It is rather neat -once you have your own domain to have some subdomains for different purposes. A usefull subdomain might for example be downloads.example.com. Any user will understand what one can expect at such a domain.
Things like this can be done with drupal 4.3 in the core or in 4.2 with the path module. In that case you can point your users to example.com/downloads and this page drupal will rewrite towards the download page. The way of rewriting these domains is rather neat, but having subdomains might in certain case have a better image or be more usefull.
The site drupal.org uses the shortcut hostname.example.com as well. For example to the documentation place, http://documentation.drupal.org leads to http://www.drupal.org/node.php?id=253 (mhhh, we should use the new clean url here!).
So how can you use this service? What follows is a short howto.
Say you own the domain example.com (rfc2606);
Say you have a drupal page example.com/node/view/nid
You want to have a shortcut to this page labelled nid.example.com
Now you have to do the following things:
1. Edit your DNS
2. Edit your .htaccess
Regarding 1), editing your DNS