Appears there’s a bug here - see the evolution :slight_smile: http://josm.openstreetmap.de/ticket/5170

Anyway, the purpose is when someone has merged highway and boundary ways and they need to be split. See http://lists.openstreetmap.org/pipermail/talk-us/2010-June/003455.html and previous posts for why boundaries, when they start off with decent precision, should not be tied to roads.