What we see at grid scale are batteries being used for quick response power while other generators are being spun up. The thing is the grid very much doesn't like it when demand exceeds supply. Systems shut down to protect themselves and you either dump loads very quickly or suffer a cascade failure (see the 1965 blackout--and note that that only stopped growing when the operators were able to dump enough load.)

Since this is in disaster territory the utilities obviously try to avoid it and ensure there's always enough to cope with any surges--which means they must have more stuff spinning than they actually need. Enter facilities like the big batteries: keeping them hot costs almost nothing and they have a very fast response when called upon. This buys the utilities time in which to spin up other generators and thus allows them to operate with less waste.