That's not the main use of grid batteries. The main use of batteries is 1) to soak up power that is otherwise not needed when there's too much of it. 2) to deliver that power back when there's too little of it. This is something that happens very often for typically short amount of times (hours). Batteries help smooth this out.
The mistake you are making is only thinking about when there's not enough power. The real challenge is dealing with the very regular situation that there's too much of it. That's energy that is wasted and lost.
Batteries improve the capacity factor of renewables (the percentage of time they are useful).
So, do electricity cables. Shortages and surpluses are highly localized. Germany for example has the problem that the demand is in the south and a lot of wind generation is in the north. So, they are curtailing wind power when there's too much wind and are firing up coal plants in the south because they lack the cables to get the power from where there is too much of it to where it is needed. When Texas had it's blackouts, other states had plenty of power. But Texas is not connected to those states by cables. So they had no way to get the power delivered. So, blackouts happened.
Long term storage is much less relevant currently and a market in it's infancy. The overwhelming majority of grid batteries is for dealing with short term dips and peaks in power generation. Most setups don't provide more than a few hours of power at best. But they can switch between charging and discharging in milliseconds and do both at high capacities.
This is why lithium ion is popular in this space. It can deliver or soak up a lot of power very quickly. You can put cells in series or in parallel depending on the use case. You add more cells to deliver more power more quickly. Not necessarily for longer. You can configure the same 1gwh of cells to deliver 100mw of power for 10 hours or 2gw for half an hour. Most of these batteries are configured for high capacity charging/discharging and relatively short storage.
There are some long term storage solutions emerging as well of course. Redux flow batteries are a good example where there's a fixed size cathode and anode and reservoirs of electrolyte that are pumped around. You can scale these by simply using larger reservoirs. They are cheap and can hold many days/weeks of power. Just add larger tanks. The caveat, is that the power delivery is a constant and typically low.