Two cases :
Case 1- example.com's zone file :
@ IN SOA dns.example.com. info.example.com. (1 8H 8H 8H 8H) IN NS dns dns IN A 184.108.40.206
How are the second and third line useful? I understand that they may be necessary in order for dns.example.com to be defined. Is there another use?
Case 2- foo.com's zone file :
@ IN SOA dns.example.com. info.foo.com. (1 8H 8H 8H 8H) IN NS dns.example.com.
In this case, is the second line necessary? Why would it be useful to have this NS record for the foo.com's zone if we are already IN the zone?
Additionally, is it necessary that the first and second line both point to the same server? If no, in which situation would they be different?