That my friend is exactly what a sub domain is. Its just an extension of your domain.

It comes free as it is free for you to make. When you create something in WHM or most other panels if you create an account with the domain bla.x.com it automatically makes it.

Its more of a way for people to have a domain free until they purchase a real one. Or people that are not really interested in the domain, as it is not really a for the public ( Ex. family photo album or something to that nature. People will not be using your company name with their site.