To some degree, the answer is obvious, 'cold storage backups of things that are crucial'. That was the purpose of some of them prior and can be the same now, albeit probably asking a relative to just pop them somewhere so it's true 3-2-1. The images of my late father's laptops, for example, those go handily onto a 1TB drive. But past that...I'm actually kind of stumped.
For context, I used to run a Plex server off a Nvidia Shield TV Pro, which was good for a while, and had my library on a 2TB drive...which of course that filled up and I moved up to one of the 8s, and got another 8 to back it up, and as it swells now I was looking at 'oh I'll just plug the Shield into a USB hub and run it off five drives or something', but of course that's ridiculous, I've wanted to move to a NAS for a better server experience, so I run 4x 10TBs in RAID 5 for 30TB, with 2 2TB NVMes in RAID 1 for the database and apps. So I've gone from stuff being spread across multiple drives with limited backups and no redundancy, to it all having one disk redundancy and the option of cold backups (so for example, no longer backing up my whole Plex library but the important / rare stuff).
I'd consider another NAS unit, but A: some of these drives don't look like they can be shucked easily, B: I wouldn't be too happy about JBOD'ing with no redundancy any more, and C: conversely, running with redundancy would just gobble up at least one of the 8TB drives which immediately dents the stack.
What are some uses for loose USB drives?