Many uses for subdomains
Many people don't realize it, but when you use www when visiting a website, you are using a subdomain of that website. For example, www.google.com, in this case google.com is the domain name and www is a subdomain. The majority of sites allow you to access them using both the sub domain and the actual domain. But it doesn't have to be this way. In all technicality, www.google.com could point to another website that is not google.com, it could even point to an entirely different server.When you think about it, in this day and age typing in www.domain.com is kind of silly.
This becomes handy when you want to host different parts of your application on different servers. Perhaps you have a members directory that requires a different platform than your main website in order to run, but you would still like people to access it under the general domain name for your site. No problem, just setup a subdomain like members.yourdomain.com and point it to the ip address of a server that is separate from the website hosted on yourdomain.com. It is just that easy.