I'm guessing it's for time sync, but any clock worth it's crystals will keep good time for quite a while. Every substation generally has GPS time sync if it is

I'm guessing it's for time sync, but any clock worth it's crystals will keep good time for quite a while. Every substation generally has GPS time sync if it isn't fibre connected, for synchrophasor measurements or accurate timestamped waveform captures for example.