I've given this some thought, it's not a bad feature, and not as useless as it first seems. It would be especially useful in the situation where developers work on multiple projects for multiple companies where these companies like to have the issue trackers on their own servers. So the developer could have all projects on their machine, but each company only has their own projects on their server.
it also allows a developer to work offline, using their local machine for issue tracking, and then syncing with the main project server. It would also allow projects to be shared between multiple devs on their own machines and linking to sync.