Why do some registrars ask for nameserver IP?

[EricM]

Hello.

I know the owner of the domain that hosts a nameserver registers the nameserver and ip with his registrar.

So why do some registrars still ask for the nameserver IP when setting nameservers for a domain?

I remember many years ago it was standard for registrar to ask for nameserver and ip, is it just some of them havent updated their system to the new ways?

If the owner of the nameserver changes the ip with his registrar, will the nameserver stop working for the registrars which ask for the ip until each domain is updated with the new ip?

Sorry if i sound confusing, i dont know a better way to phraze this.