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Edit for why: On-demand power is expensive. "Spinning up" additional power during the day is more expensive than having certain types of power generation running all the time. In theory, pumping power into a battery at night during non-peak hours will cost less than consuming directly off the grid during the day.
I'm forgetting all the details of energy economics, but IIRC there would also be far less emissions by having consistent load on the grid all day long. Something about the ones we "spin up" also being the worst for the environment.