what is purpose of Add a New Domain function

Hello.
in centos web panel. to create a new domain , we have to create new user account which will add the domain for that user as well.
but what is the reason for Add a New Domain in Domain tool set ?
is that for any other domain for same user other then the was created in New user Account ?
why first domain never gets listed in List Domain section ?

any thoughts ?
Thanks

One user can have many domains and those domain's public_html should be inside the username folder
example a user named john, he has  3 domains, those public_html folders should under a folder named john/domain1
So that when a user login using his SFTP, he is restricted to access other folders which are belong to other users