There 3 things for a domain to work with a web hosting. Many do not know the existence of DNS. DNS is actually important. It works as a directory to tell your visitor to go to the website or deliver your emails. Without DNS, your domain and web hosting will not work even they are valid or running.
Website hosting and email server are recognised and talked to each other using the IP address. An address numbers, e it an IPv6 or IPv4. Numbers are difficult to remember and it is not practical in the past using IPv4, thus there is such thing known as a virtual host, so many domains can use the same IP address.
This is where and how the DNS works. It helps to translate a name to numbers Like what I have said, DNS direct traffic for email or website. Without DNS, the name cannot be translated or resolved, everything will stop working on the internet.
Domain names registered with the registrars require 2 or more DNS. I personally prefer they are from different networks and GEO locations. So, if one DNS is down, the others are not affected should its network is having troubles. If you have 4 DNS but they are set up on the same network. It will defeat the purpose of having 2 or more DNS.
This article tells you DNS is relatively important but usually neglected. Hope it helps.