It's not done in practice because we don't actually have that much spare renewable energy capacity. Whenever renewables power 90%+ of current usage it tends to hit the news. It's not common. 100%+ is pretty much unheard of.

Pretty much every MWh generated by green energy these days is just a MWh of natural gas extracted from the earth that is not burned.

This is a hypothetical solution to solar and wind routinely overproducing what batteries and pumped storage can keep up with.