If the numbers work out (which, I suspect they don't), putting batteries at substations could provide continuous power to customers when the substation is isolated from the grid by a wiring fault. That shouldn't take months to fix, but sometimes it takes days (in my area, it's usually trees falling during storms, and the roads are dangerous during such times, so it takes some time for repairs). Might be nice anyway, and help with grid stabilization when the wiring is intact.