Comment 3 for bug 1917099

They are needed for DNS recursion. If parent.example.com is zone transferred to other servers (lets say ns4.website.com) and then a client queries ns4.website.com for something in child.parent.example.com then the client will not get an answer because ns4.website.com does not know who is authoritative for child.parent.example.com due to not having NS records for that zone. DNS recursion, in my understanding, would work like so:

root nameservers -> .com nameservers -> example.com nameservers -> parent.example.com nameservers -> child.parent.example.com -> SOA record.

I will submit a patch via Gerrit, apologies.