Role of Authoritative NS record in bind9 compared with Non-Authoritative NS record in Registrar

Question

Whenever we register a new domain name from a Registrar, we need to tell them what is the nameserver by providing them the dns server IP address.

But in the same time in the zone file of our dns server (I will use bind9 in this case), we also add the RR like the following:

abc.net.    IN NS ns1.abc.net.
abc.net.    IN NS ns2.abc.net.

which is the authoritative record.

1)So what is the purpose of having the authoritative record of NS in the zone file of abc.net, since when one trying to resolve let say www.abc.net, the TLD .net already tell us they are in ns1.abc.net and ns2.abc.net ?

2)Since from the delegation concept point of view, NS record only need to be stored in the parent of the zone, meaning root server NS tell us NS of .net, .net server NS tell us the NS of .abc.net and .abc.net NS resolve the dns query. so in order to resolve www.abc.net, there are only 3 set of NS IP address we need to know:

1. root server NS that tell us where is .net, .org, .uk, .it...etc. root server NS IP is store in bind9 named.ca
2. .net NS that tell us where is .abc.net, .xyz.net, ...etc. .net NS IP is stored in root server NS record.
3. .abc.net NS that tell us where is www.abc.net, mail.abc.net,...etc. .abc.net NS IP is stored in .net NS record.
4. so since we don't have any www.subdomain.abc.net, we shouldn't need to store any NS record for .subdomain.abc.net, isn't it ?

3)What will happen if I don't put the NS record of abc.net in my abc.net zone file ?

4)What if I have NS record in my zone file but not in the Registrar? which one will be the resolver take ?

There are a detail answer here, but I still don't understand the meaning behind it.