Name servers are required because they store the DNS records that connect a domain name to services such as websites and email. When someone enters a domain name, name servers provide the information needed to locate the correct server.
Most domain names use both primary and secondary name servers to ensure that DNS information remains available even if one server is unavailable.
For a more detailed explanation of DNS and name servers, please refer to the following resources:
https://atlarge.icann.org/topics/dns/background
Provides an introductory overview of DNS concepts, domain names, and DNS infrastructure.