In this case, it's less about discoverability, but more about long term archival. Will daemonology.net continue to exist forever? Arxiv.org might perish, but I am sure the community will make sure the data will be preserved.
If the course website was even on the open web to begin with. If they're in some university content management system (CMS), chances are that access is limited to students and teachers of that university and the CMS gets "cleaned" regularly by removing old and "unused" content. Let alone what will happen when the CMS is replaced by another after a couple of years.
https://wiki.archiveteam.org/index.php/University_Web_Hostin...