In most web hosting companies, you can host unlimited website in single account. This is true with my own host, the HostGator.com. I can actually create as many website as I can with no limit. Addon domain will accomplish the task of adding new website to your existing account.

As our site grows, we really feel the need to expand. You have a new whole fresh idea appropriate for your successful site and to its audience. To add new feature to a very active site may lead to disaster, specially if the feature you wanted is really huge. This is where the need of a subdomain arise. With the subdomain, you will not mess up your stable database structure, and you don't need to go offline while in development.