for a limited period - say 24 hours. They're designed to help you when you're transferring a name from another host, and want to start work on a website in the interim, while you're waiting for the name to come over from the previous host.
You can use any name you like, because unless the name is transferred in within the specified period the reference will disappear.
I also have a reseller account, and the way I handle development is by running sub-webs on a special domain name with password authentication. Nobody but me, or anyone to whom I allocate a password can see the work in progress.
It works perfectly - I've been doing it for years.