Family interruptous. Which is very welcome while I'm in Stage 4 Coronavirus Lockdown.
Could you explain it a bit ... why once off?
I do that when I want new files in one place, separate from other media files, usually because I am doing work on or evaluating them. Being in one folder I can easily filter down to just those files in MC, or use a Files View to drill down to their location, or use the MC Explorer function to manage them. That works regardless of any relationship, or lack of it, between the files.
There are a few reasons for managing new media files that way. One is that I am setting up a test of something in MC, and want a new set of files to test on. So I copy some files to their own folder, then import just that folder. When I am finished with them I can quickly delete them from the Library, even if their tags now put them in different Views and not together. There are other ways to handle this, but I gravitate back to this simple method often.
Another process I use is to review new audio files, and I want to do that in MC. Then if I am happy with the files I can just move them using the RM&CF function in MC. If not, I can just delete the folder, even just using Windows Explorer, and MC will remove them from the Library. But my preferred approach is again to use the MC Files View or MC Explorer.
I used to do that a bit with downloaded Podcast series, but not so much anymore. I still do that with images, if I haven't culled them with another application. Or just to decide where they should reside in the Library.
I also use this method with Work In Process videos I am creating, from my action cameras or the mobile. I often import incomplete Premier Pro project folders I'm working on to take a look at them in MC, or to bookmark where I'm at. Those will never be retained in the Library, as only the final result is kept, and it will be stored with other media files.
Maybe it is a bit of a strange way to use MC, but that is what I do, and I find it saves me time and gives me better control.