Before solar, utility companies would install baseload plants (nuclear, coal, AFAIK) which output power at a constant rate for cheap. The idea is that your baseload plants would provide power right up to the trough of the demand curves.
Solar causes that trough to deepen but doesn't get rid of the peaks (see duck curve). That forces power companies to close down baseload plants, but add peaker plants (usually natural gas) which are generally more expensive to operate.
This is where grid storage enters for a green grid. It brings two benefits, it can raise the demand trough making baseload plants attractive. It can also drop the demand peak, decreasing the need for additional peakers plants.
The problem is that we don't have a lot of viable storage options. Best case for a region is having a lot of hydro available. Pumped hydro would make sense, but there aren't many places where you can install it (it requires a lot of water, land, local buy-in). Batteries work, but have cycle limitations. You could do flywheels, but they are expensive for the power stored. Heck, one even seen "fill a pit with gravel, heat it up, boil water during peaks to generate power". Which may make sense, particularly for colder regions where that heat could also heat housing.
Electrical utility companies manage this with two major levers:
1) Standby power plants, that are running but not producing at full load. The plant load is actively adapted to the monitored needs of the grid.
2) Hydroelectric storage, to soak up excess production, where hydroelectric dams with secondary reservoirs are available.
Grid load management is very much an active process, not a passive self-correcting one.
When the net frequency increases so does the rotating speed of all turbines. The inertia of those turbines is one way how excess energy is buffered.
While electronic componentes are making big steps in that direction, "classic" mechanical generators are still superior for this as they are running very constant.
That's why lots of people fear that solar panels with their sudden power generation (as in "clouds are gone, let's bring it on!") might be a danger to the grid as they are fluctuating quite a bit.
So to balance the supply of electricity to customers you need a way to make sure there is an not too much and not too little.
Many years ago in Britain, it was predictable that at the end of the TV show Eastenders, people would all go to turn on their kettle for Tea. The surge was so big that someone had to monitor when the programme ended and initiate the additional supply of electricity to meet demand. That supply even had electricity from France as a backup.
I recall an interview where they had that energy available from a Hydroelectric Power Station where they could release the water and get an instant source of electricity.
All for tea.
Edit: fixed wording