A good to way to take unreliable sources of energy and convert them into reliable ones. You could probably do sun and wind to biofuels.
In the old days, you would build ONE power plant to meet your needs. It would ramp up and ramp down to meet the instantaneous electrical demand.
Now you need TWO power plants to meet the same need. Wind and solar make what they want. You cannot ramp them up or down. So, in addition to Power Plant #1 (wind and solar), you must now build expensive Power Plant #2 to fix that problem — pumped hydro or batteries.
Does doubling your capital cost make sense to you?