so-called "gravity batteries" is pretty much exactly a dam with a mini-dam/reservoir at the bottom. When there is an excess, you run the motor to reverse the waterflow to pump uphill into a highe-elevation water retention pond/mini-dam.

This also helps reduce the amount of outflow water "lost" due to high-demand. Since you could take almost a day to fill the bottom reservoir and spend "wind"/solar to pump back the "lost" water downstream back into the higher-level reservoir.

Even if things are inefficient wind/solar are "renewable", so you can keep "wasting" excess to replenish the dam and still make enough money back ( think in-terms of drought, flooding, windy, sunny, cloudy, etc ) you can basically keep the high-output "system" always topped-up with water. And still supply water + electricity as it is needed. There is no "downside".

Not everyone agrees. So opinions can differ.