Durham_e - 10:31 am on May 11, 2008 (gmt 0)
The reason is that the subdomain may be hosted on a server that has limitless space and bandwith, say for big file images or video. The main domain being on a another server say in a country adventageous for serving the actual audience. Think SEO etc. It sounds like a good scheme and I was about to ask a question on the best way to set up such an arrangement in terms of DNS etc.