They power the machines used to gather materials. They power the machines that move those materials around the world to be turned into goods. They power moving those goods around the world to be sold. They power moving them again once they’ve been sold. And if we’re really lucky, they won’t use any more at that point.
The electricity you use. The gas in your car. The gas you use to heat your home or cook. The gas the Amazon van uses to get stuff from the warehouse to your door. The gas used by the semi truck to move stuff from one warehouse to another. The gas used by the cargo vessel to move stuff across the sea. The gas used for the mining equipment for the raw materials to make stuff. The gas used in the machines to turn materials into stuff.
Hell, the gas used to harvest crops and move them around and keep them cool, if need be.
Yes, we can and should be working on ways to divest from fossil fuels at every opportunity, but even if everyone was perfectly on board it wouldn’t happen overnight. It’ll take a few lifetimes at best.