doubt nameservers

What is the purpose of putting the nameserver ns1 and ns2? if cwp doesn't add them in that ns automatically or nameserver's own cwp is used 

For example, what would happen if I don't put anything there? or do I put something that doesn't exist ... or what would happen if I put those of cloudflare chloe.ns.cloudflare.com and kurt.ns.cloudflare.com?

You don't have to use your server's DNS service; because the name server section is a good solution when you just want to keep the DNS records of your domain names on the server. If you are using Cloudflare, there is no point in filling in the name server section, you can leave it blank.