Effectively the time when it is useful is "when we don't have cheaper alternatives yet". We still should strive to make renewables and storage the cheaper option though.
To get really pedantic our 20th century understanding of power and energy are exactly backwards colloquially from what is really provided. Power is energy over time. The "power" infastructure was actually largely an energy infastructure with the exception of say hydro electric dams - you can only burn fuel once no matter how clever your ability to extract it. Meanwhile "renewable energy" provides power over its period of existence.
