Is explained in the article: Conventional Generators have enormous energy stored in their rotating parts, and the grid ties all these generators together as if

Is explained in the article: Conventional Generators have enormous energy stored in their rotating parts, and the grid ties all these generators together as if they were on the same shaft. This maintains the phase and frequency of the grids electricity by providing "synchronous momentum". However this synchronous momentum is lost when the mechanical generators are switched off. And so giant batteries and inverters are used instead to provide this critical function. To put it another way, the giant batteries are not used to power the grid, but instead used to lower the AC impedance of the grid, eg provide synthetic inertia.