ipv6 dhcpv6-pd no-dns

Under what circumstances would I need to use no-dns when using SET commands?:

For example when I

set interfaces ethernet eth0 dhcpv6-pd 'no-dns'

or if I leave no-dns out like

set interfaces ethernet eth0 dhcpv6-pd

The result for me is the same --- my ISP provides me with a ipv6/128 address

So what are the circumstances that a no--dns directive must  be used?