The battery's value comes from its short response on the time scale of milliseconds. Conventional power plants are mechanical systems, and because of that it would take orders of magnitude more time to start them up for backup power generation. For that reason, at all times, coal power plants generate a little bit more power than there's demand for. If some other power plant trips, the frequency drop is detected, and the surplus kinetic energy of the turbine is immediately transmitted to the grid to stabilize the supply. However, there's the cost. The power plant has to generate surplus power at all times just because it can't change its state fast enough in case of emergency. The battery technology, with its response times typical for chemical/solid state systems, slashes the cost of conventional power generation by making this surplus backup power unnecessary.