Not all vampires in the land of the dead are bloodthirsty killers that want to take over the world. Some are satisfied with immortality and want to spend an eternity without strife in seclusion away from contant war with the humans.
The size and number of knots in hair, tied around neck, or hanging from belt could signify family, importance, worship, guild affiliations. Being able to translate the meaning of the size and placement of knots can give you a a large understanding of who somebody is.
May be used in a more barbaric society. Where in cities peoples clothes and crests show who they are, barbarians have no need for this so use knots.