It's a form of capability-based security and it makes some forms of sandboxing or access control trivial (i.e. removing rights from processes). If you want to block a process from accessing a certain subsystem, just rmauth its session token to the server.
I remember seeing a demo of a very early version of HURD in 2002, and I thought this was the most interesting thing about it. I think in the demo, Marcus Brinkmann showed how in the midst of editing a file in vi in user mode, you could open another file in superuser mode, do an edit there and then go back to the old file in user mode.
