You remove sites when you want to permanently delete them from the system. As an alternative to removal, you can disable sites, which prevents the use of control panels and services without removing them.
To remove sites:
The site, its virtual services, and its DNS zone information are removed from the system, and the action is listed on the Action log.
If the site used IP addresses for services, those IP addresses become available for reuse. However, they are placed at the end of the available IP address list in their scope; they would be the last addresses to be assigned.