The usual reason for this sort of design is so you can support loading a new configuration without having to restart the whole thing, if the new config might need you to do additional things that require privilege (maybe bind to new low ports, or read key files).
I would expect restart to bind to new ports. It's not something that should be supported on-the fly (it's good thing, of course, but not at expense of keeping root process). Reloading key files is useful, but I think that it could be implemented with other means (e.g. reload program which runs under root, could read those files and pass them via some kind of IPC).
