Standard seconds are defined based on measurable properties of a cesium atom. The historical definition of 1/86400th of a day doesn’t work for science if the duration is inconsistent.
For example the statement:
Earth’s Days Are Getting 2 seconds Longer Every 100,000 Years
becomes self-referencing and loses all meaning without some other reference point.
In clocks like this, the “set time” is often irrelevant. It’s more important to know exactly how much time has passed since the last time the clock was “checked.” If you’re running a radio transmitter at 6ghz, that’s 6 billion cycles per second. If you synch your transmitter to your clock once per second, it had better be accurate to the billionth of a second.