Hydro dams use high-pressure water under gravity to drive turbines connected to generators. They are effectively mechanical batteries in that we can use electrical energy to pump water into the dams when demand is low and then release it when demand peaks to provide the additional electricity when needed.

Battery storage is needed to reduce the need for baseload generation (which is the minimum needed to be generated consistently) and to use the off-peak times to store the energy that can be used during the peak times to effectively create a relatively flat demand to use the least amount of resources to generate the required amount of electricity.

image credit: EnergyVault.com