This seems extremely common for users to do and I’ve wondered about it also over the years. People will store important emails in the “Deleted Items” in Outlook and files in the “Recycle Bin” on Windows and the “Trash” when it used to be called that.
I think because it’s easy to do? Even though it’s dangerous. I assume they are attempting to use these places like an archive. Stupidly, poorly, and with no thought behind it at all, but I guess that’s what is going on. At previous roles I’ve had to make announcements via email and Slack that, “We do not back up the Windows Recycle Bin or Deleted Items in Outlook. Do not store needed or important items in those locations. They will be deleted and will not be available for any recovery or restore.”
It worked better than nothing, but people still did it anyway.