Since wind and solar power generation is essentially free, why do they even turn it off?
If solar and wind is free why isn't everything running on solar and wind? The reason it that is it not free. Wind has moving parts. They wear faster under load so free wheeling or stoping the blade reduces wear. The solar pannels themsleves wear at about the same rate whether producing or not (they do wear out) but not producing reduces wear on other components - especially the inverters. Solar pannels produce DC and it has to be converted to AC. Those inverters - especially huge comercial inverters - do wear when running and do not wear when turned off.
Solar and wind are not free. In fact they are the two most expensive ways to produce electricity we have.
EDIT: my comment on solar pannels applied to stationary pannels, not trackers. Solar pannel wear is directly propotional to total solar flux. So if a tracker is turned away from the sun to turn it off then the wear on the pannel itself can be mitigated.