You don't have to do that... NextDNS ultimately uses Cloudflare DNS under the hood, so just point yourself at 1.1.1.1 and you're done but it will block nothing.

If you want a DNS server to block things, know that the definition of what to block is subjective and some people may disagree with what you want to block.

For that reason you get an account, which allows you to have a configure, and that needs to be resolve to you... so at some level an identifier of the configuration is needed. This is the id.