While the biggest peak is around midday, the second biggest is in the evening (most people are home, cooking, watching TV/listening to music/playing video games/etc; or in restaurants, clubs, cinemas, etc) which, depending on location and time of year, can easily be after sunset (e.g. half the year in the Northern hemisphere for sure). You still need enough power to cover that, especially if it has been a cloudy/rainy day, or week, or month.
> Non-renewable plants are the ones that need buffer. Solar, wind, hydro etc can all be connected to a grid with zero instability- you just unplug them if nobody wants the power. Non-renewable plants have slow ramp speeds- they need the buffer in order to follow a changing load
And this is so easy and foolproof to do, just check out the Iberian power outage.