A few years ago, a dispute between Kosovo and Serbia caused the entire European grid to drift away from 50.000Hz down to 49.996Hz. Millions of microwave clocks across the continent ended up 6 minutes late: https://hackaday.com/2018/03/09/europe-loses-six-minutes-due....

Clocks used to be able to use the 60Hz cycle to track time, and grid providers would run slightly slow or fast ("time error correction") to get back into sync. A leap second would just be part of this.

I believe in the US this error correction has been discontinued in the East and in Texas, but is still done in the West for some kind of non-clock "inadvertent interchange" reasons I don't understand.