With all the advantages renewables have (please use and develop them!), there are some instances where they can't reasonably be used.
For example, I live in a city of 5 million people that gets very little direct sunlight (weather is cloudy most of the time + city is located 60° North), has highly irregular seasonal winds, has rivers too small to make hydro make sense on such a scale, and barely has good hills for pumped hydro. It is almost exclusively powered by nuclear energy, because under these circumstances, there's barely a greener alternative.
There is also the need to have a backup power source for most solar/wind installations, as through some parts of the year they can only provide negligible output.
Finally, some regions might require temporary power - either due to such seasonal downtimes, or because main grid has failed. For that, Russia and China operate vessels with onboard nuclear power plants to source energy through these periods - and then move on to help somewhere else.