This is from times where Unix & co only ran on a corporate server and cubicle slaves accesed it via thin client. There was /home/alice-abbey /home/bert-branson on one disk of the server, ‘/’ root on another, with less storage and more performance. And often /usr on a third. While /-root has to be locally, everything else can be managed however you’re funny, even nfs shares. But historically, /mnt was for temporary mounts. I think the /run/user/usernane/diskname came up with xdg, it’s where graphical filemanagers find disks.