Oh, almost I forgot.

Some manufacturer of 8051 derivative has decided to complements the watchdog timer.
Normally, the watchdog timer is initialized so that it will time-out and will cause a processor reset in the event that the processor loses
control. In the event of a crystal or external oscillator failure, however, the watchdog timer will not
function and there is the potential for the processor to fail in an uncontrolled state. The use of the
oscillator fail detect circuit forces the processor to a known state (i.e., reset) even if the oscillator stops.
That is what 80C390 manufacturer likes. If the watchdog timer clocks in, comes from ring oscillator or something similar (and not use the same clock signal like microcontroller core), obvious the above improvement is worthless.