I have multiple drives that started out as their own os. Each of them has a Dropbox folder in the standard location. Each of them has a different set of files in them (I deduped at one point), with some overlap of different versions. I no longer use Dropbox, so none of these are synced anywhere.

They don't need to be in my case, I'm only using them now because of existing shortcuts and VM shares and programs configured to source information from them. That doesn't mean I don't want them backed up.

Same for OneDrive: Microsoft configured my account for OneDrive when I set it up. Then I immediately uninstalled it (I don't want it). But I didn't notice that my desktop and documents folders live there. I hate it. But by the time I noticed it, it was already being used as a location for multiple programs that would need to be reconfigured, and it was easier to get used to it than to fix it. Several things I've forgotten about would likely break in ways I wouldn't notice for weeks/months. Multiple self-hosted servers for connecting to my android devices would need to reindex (Plex, voidtools everything, several remote systems that mount via sftp and connected programs would decide all my files were brand new and had never been seen before)