why do we need to do that? Is this how cpanel works?
No, this is how the real world works
Standards mandate that your server have a hostname, something like sub.domain.com . When you're managing or running multiple servers, you WANT sub.domain.com , not just domain.com.
In addition, most people will bounce mail sent from improper server setups. In your case, with just domain.com, this is an improper setup, and you can expect mail to be bounced.
Even if you only have one domain, and you're not setting the the server up for mail, stick with the standards. Yes, you can use ns(or ns1 or ns2), NO you can not use www, because www should point to a user, not a server.
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