Purpose of an NS record for the current zone in a DNS zone

You may need to do some research about DNS and it's records. The First line in the first example is the Start Of Authority. It provides basic details such as the master nameserver, the administrative email address, the zone serial number, and timer options.

Second line is determining that the server is indeed an authoritative server for the zone. Meaning it has authority to respond for the domain. This is the purpose of NS records.

The following is the glue record associated with the nameserver. Since the NS record is in the same zone as the domain you are looking up, it needs to determine the IP to hit in order to preform the look-up.

In the second example, it follows the same path, except, since the nameservers are in a different zone, it does the look-up inside of that zone instead.

With DNS, it is a systematic process to determine you got the right records for the right things. That is why you will often see multiple lines as it steps through the appropriate records.