Subdomains are the perfect tool to separate your website in sections in an SEO friendly way.
When for instance you have your website hosted on your main server, but your support pages are hosted on a different one, creating a sub-domain such as support.mydomain.com can be very useful and won’t distract your visitors from the fact that they went onto a different server.
Frequently it is necessary to create simple addresses that direct to different parts of your web content so that your users can better remember them. Subdomains allow you to do this.
For example, you want your customers to continuously visit your forum, but the URL "http://www.example.com/scripts/forums/index.html" is just too much to remember. With a subdomain, you could create "http://forum.example.com" that will take your users to the "http://www.example.com/scripts/forums/index.html"address.