The electrical energy production in a grid at any given moment needs to exactly equal the demand. There is no energy storage in the grid. You need to match supply to demand.
So why doesn't the grid fall apart when you turn a light switch on? Well, it turns out we have tonnes, literally, of fly-wheel batteries connected to the grid. These are the generators, which have momentum. When a new load comes on line it will start taking energy out of the fly-wheel batteries, that is slowing the generators as it steals it's momentum. Of course, the generators have finite momentum, so this will continually slow the generator to a stop. You need to get demand and supply back to match, the grid needs to be stable. To regulate the grids power supply to match, we simply maintain the speeds of the generators. If a generator slows down, a control system notices, it drives more power from the prime mover (turbine usually) to the generator, and this additional power supplies the new load. Vice versa for a load turning off.
So the European grid has been underpowered, due to serbia/Kosovo not regulating their generation, so a bunch of energy has been taken out of generators across the continent and slowed then all down by a very marginal amount, although the power bill for this lost energy for such a small frequency loss is actually quite substantial, probably about ~$10 million USD. No one wants to be the one to correct the loss, even if they had the capacity, to speed back up every generator on the continent on their own dime for someone else's failure.