Renewable energy sources like wind and solar are intermittent (one can't create electricity using solar at night, or using wind when the air is still, after all

Renewable energy sources like wind and solar are intermittent (one can't create electricity using solar at night, or using wind when the air is still, after all), whereas furnaces can, effectively, run whenever there is demand. Electrolysing rust into iron can be done when there is excess renewable energy available, and it can be burned when there is a deficit. In other words, it's a chemical battery.