With DHCP-assigned IPv4 addressing I assign firewall rules and thereby effectively permissions to certain privileged boxes. For example some devices, selected by their IPv4 addresses, are rate-limited, they only get a restricted amount if downstream and upstream, so that they can't hog the network when I'm trying to work. But at the moment they can get out of this restriction by using IPv6. Yes I know I could block IPv6 to/from them, but that is philosophically rubbishy.