There are reasons you would want to use fossil fuels and use renewables to reverse the process. Fossil fuels have high energy density, can be transported and stored with minimal losses, and consumed anywhere anytime. There are lots of things that are difficult to directly electrify like say planes. You can't simply stick a bunch of batteries on a plane to replace a jet engine and its fuel tanks. But building an excess of solar panels on the ground to mop up the CO2 equivalent of the jet engine's exhaust is simply a matter of cost. E-fuels are basically just really dense chemical batteries.