A website default page is created after virtual server was setup using Virtualmin, to help server administrators to determine if a newly created website is working as expected.
A properly working top-level server or a sub-server should always match the domain name on the default page with the same domain name used on the URL.
Server administrators can disable virtual servers for whatever reason. For this purpose a website default page will reflect it as well.
Also, server administrators can configure to use website default page as web-server custom error pages, by passing as a query error parameter with error code, i.e.
?error=404, which will display an error with clickable link to error description.