"Volume could not be unmounted" in disk utility first aid

Question was posed several years ago but doesn't seem to have received replies. So, again, please?

I have an older iMac ("late 2015" maybe) running the latest OS, and this point has perplexed me for years (through several iterations of OS).

If, for example, I lose patience with "Pages not responding" or with my screen having an app diminish in slow, stair-step fashion, I do a safe boot and go to Disk Utility for First Aid. (This may not be the solution to my problems, but it is what I do.) What always happens is that the first aid reaches line 3 or 4 and reports "Disk could not be unmounted."

It then moves quickly through several steps in its process--likely too quickly to be doing anything--and works its way to the end of the list. Reports things are okay. My questions are Must the disk be unmounted for the first aid to be done? If so, why doesn't it unmount? What can I do to unmount it? Or is the fact that it wouldn't unmount a good thing?

Any/all help sincerely appreciated.