I see that almost all registered domains have at least two authoritative name servers configured (and for most of them, exactly two), and that using two servers provides redundancy against any kind of server failure. Technically there's no restriction on number of auth NS servers, so a single NS also works.
However, for some zones, there are already multiple addresses for one server configured, like
ns1.qq.com (4 IPv4 addresses, auth NS for
qq.com.). And for some other zones, the name servers are distributed using techniques like anycast, like
ns3.cloudflare.com (one anycast IPv4 and one anycast IPv6).
In those cases, why would one still configure multiple authoritative name servers for a zone, when one NS entry can already provide redundancy against physical server failure?