We also need hardware write-protect in storage media such as USB flash drives. Otherwise any system the USB is inserted into can compromise the USB. You usually think of USB compromising a computer, e.g. Stuxnet, but it can also work in the other direction.

Back in the old days of 1/2" mag tape reels, the rule was "no ring, no write". There was a physical ring that needed to be present before a tape drive would write to a mag tape. This was also relatively failsafe. If the ring fell out or was removed, for any reason at all, the data was guaranteed to be safe.

This same mentality carried over to floppy disks. It was possible to protect media in hardware.

But in the interests of saving money, hardware write protect circuitry is quite passe these days. As are hardware switches of all sorts.