Power plants must have sufficient capacity for periods of peak demand. This is not negotiable. Solar and/or wind power arrives at unpredictable moments. There may be none at all, at a given moment. The power plant must be ready for that and be able to deliver power.
Laws that mandate power plants to buy electricity from solar/wind units are stupid laws. Electricity is not like water. Those laws are a disguised subsidy to those with wind/solar power, paid by the other users of electricity.
https://www.wind-watch.org/news/2010/04/08/wind-power-is-a-complete-disaster/
“There is no evidence that industrial wind power is likely to have a significant impact on carbon emissions. The European experience is instructive. Denmark, the world’s most wind-intensive nation, with more than 6,000 turbines generating 19% of its electricity, has yet to close a single fossil-fuel plant. It requires 50% more coal-generated electricity to cover wind power’s unpredictability, and pollution and carbon dioxide emissions have risen (by 36% in 2006 alone).”