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.