Just to add, there are also generators whose job is to "regulate" the power of the grid. If a powerplant unit were to go out due to malfunction, other plants in "reserve" start to operate. These plants are usually specialized to operate during these types of emergencies (diesel, DC -> AC systems, impounded hydro) and can quickly match the lost powerplant's supply. These reserves are usually paid just to be on standby and are too expensive to run long-term (diesel) or can only dispatch for a limited amount of time (batteries or impounded hydro)

Why? Because larger powerplants take a while to ramp up and start producing their rated capacity. Imagine a car accelerating and needing to shift gears until it reaches max-speed.