50 Hz grid frequency is (usually) a precisely maintained standard which is used for long term (days, weeks, months, years, decades) timekeeping.
Through the day it can vary slightly as loading varies, but this is averaged out over 24 hours, with great precision.
For example, my UK electricity supply is “Economy 7” meaning my metering switches at night to a much lower price for 7 hours, during which I run e.g. an immersion heater for hot water, dishwasher and washing machine.
This switchover is performed by a marvellous electro-mechanical clock, locked to grid frequency, which has kept perfect time for at least 34 years (since my house was built). It has clockwork spring backup to keep it running through power cuts, and I’ve had plenty.
The electricity meter itself has been replaced three times, and is now a solid state digital device with barely readable faded LCD displays, (and I am being pressured to have it replaced with a “smart meter, but that’s another can of worms in the UK at the moment.) But the Economy 7 switch has never been. When I questioned the technician who fitted the last meter replacement he explained that no digital device had the reliability or stability of the electromechanical clock.
Certainly no inexpensive quartz oscillator, nevermind backup battery. The electronic meters have to be replaced every 15 years if they even last that long. The old spinning disc meters last pretty much forever (my parents one is at least 57 years old, probably much much older, and has never been replaced).
The UK grid is disconnected from mainland Europe, except for DC interconnectors which are converted back to UK AC frequency, so we are not suffering this frequency loss.
You can see what’s going on on the UK grid at e.g. http://www.gridwatch.templar.co.uk including how the frequency can drop at times of peak demand, and how this is corrected later. It was interesting to see how we survived our recent cold snap and gas shortage, just, thanks to coal, nuclear, a lot of wind, backup diesel generators, and major industrial users being severely rationed or shut down. Gas was severely rationed for power generation to keep domestic supplies available for heating and cooking. Our gas storage facilities are out of action and, ironically, tankers of LNG from Russia have been helpful during the crisis.
So yes, mains frequency is very much an important time source here. After every power cut I have to re-set at least my cooker, oven, microwave, combi-oven and a couple of electric heaters because all their clocks depend on it.
Yes we also have an atomic clock time signal broadcast on 60 kHz, but that could be an expensive thing to add to consumer devices for perhaps little benefit. That said, all my clocks, and one wristwatch can lock to this, and other such broadcasts. Time and frequency MSF radio signal - NPL