It's a nationally recognised scheme now jimbob, I'm tempted to say probably international.
The problem with just having wife/husband etc is that time is of the essence in situations that require the use of this number. I still have my ex husband in my phone as husband but that's due to a standing joke (I'm still in his as wifey
It's much easier for the Emergency Services to search under one word as opposed to going through the whole contacts list. Not everyone is married, or has parents etc or even someone they have to contact in such a situation.
It's a "covers all bases" heading really.