In designer you have preview but I often want to look at the published site on my phone, tablet and different browsers. If I under development have shared the url to my not-yet-ready-to-be-seen-changes, client sees all the strange things going on when he visits that url and wonders whats going on.
So to avoid that I would like to have a pre-staging domain.
Maybe this could be handy when there are more then one person working on a site aswell. Designer ships first version to client, gets feedback, gets back to work, publish on pre-staging to his collegeas for review and developers to do their stuff, then publish on the shared url to client. When site is ready custom domain is attached or the site is exported.