I think CWP was created like alternative to cPanel. You can see it in ports used by panels, features provided by CWP (cPanel to CWP migrations, etc), account structure, log files location, user level panel, etc.
Originally, cPanel divided domains to "main", "addon", "parked", "subdomains" and used different options to manage the domains. So I think CWP just copied the mechanism from cPanel. Now, cPanel tries to simplify the domain management and most likely we will see the changes in the future releases of CWP. Actually I see such changes in actual releases of CWP.