Newbie question about registrar DNS servers and NS records

[RainyCity10]

I inherited a Bind DNS server set up for a company that runs a number
of web site. I'm in the process of cleaning up the zone files and
adding additional slave DNS servers and I haven't got my head around
NS records yet. When a domain is registered you specify what DNS
servers will be providing the zone files for the domain. If that is
the case why are there NS records in my zone files that point to my
own DNS servers? Is that information ever used?

Thanks