Necessity of an external watchdog timer
A watchdog timer can guard against hardware bugs in buggy piece of ... cutting edge microcontrollers. One that we recently used, from a famous brand, had I/O pins that occasionally missed their interrupts, sometimes did not start up correctly, and where the integrated watchdog sometimes failed to reset the system in a known-good state.

This did not show up until we started with long time reliability testing, and it was easier to add an external watchdog than to change the microcontroller.

If you have more than one IC on the PCB you may also need an external reset-IC or voltage monitor to make everything boot up reliably. Many of these can also serve as a watchdog.