Sub account will create a new account like your current account, with the same default contacts and default nameservers. The sub account will have default Name.com pricing and security settings.
Why should I use a sub account?
Sub accounts are good for those that want to register domain names for others and then move them into other accounts for their customers while maintaining the ability to help manage the domain names.
Sub accounts are also great for setting up different billing profiles for specific domains or products.