An account is less identifiable than the information any DNS resolver will already have regardless of whether there is an account or not. A DNS service will necessarily know (1) Your IP Address (2) The domains you request. Using a DNS service, nextDNS or otherwise, involves some trust. Don’t let not requiring an account give you a false sense of anonymity, any DNS service can log your queries.
As to why NextDNS uses accounts, it is for the more advanced/custom features it allows. (Optional) Logs, customization etc.