[renaudg]

A typical use case for it is to dump Amiga disks, which are physically 3.5" disks but whose 880KB format is unreadable by a standard PC floppy controller.

[duskwuff]

And to preserve old copy-protected floppy disks which used strange formats and/or deliberate errors to thwart copying tools.

[actionfromafar]

Even some PC floppies can't be read by the cheap USB-floppy drives on Amazon. They typically expect exactly 1.44 megabyte floppies. Format it slightly differently, and those USB drives can't see it, even if the floppy would have worked just fine in any "real" floppy drive connected to the floppy interface on the motherboard.