I guess MS envisioned it as a digital replacement for the physical suitcase of documents you'd bring to/from work.

The whole computer was originally visualized as a digital office replacement. That's why you have the "desktop," like an actual desk top surface to work on. Files had icons that looked like papers, folders looked like the tan file folders you'd store in a filing cabinet. Plus a slew of other office-related parallels.

The briefcase was just a continuation of that digital theme. Office workers would bring their work files home in a briefcase to work on later, then bring back to the office the next day. Microsoft tried to digitally replicate that by creating a briefcase folder that would automatically sync your files to a floppy disk, so you didn't have to do it yourself. The Internet kinda ruined that concept, though. Now you can just email yourself files, text them to yourself on your phone, or store them in a cloud service to edit live on the site.