Using fat for fuel is generally a more stable source, steady fuel for aerobic metabolism. Dumping glycogen/glucose (depleting the stores in the liver) is more like a sudden burst source for anaerobic metabolism.

I can go for a long steady ride burning fat as the strategy for most efficient steady power production, but I can dump glucose from the liver for a sudden (and somewhat inefficient in the long run) burst of power to sprint away from a charging bull that escaped onto the roadway.

The body can completely switch to ketone metabolism for fuel during famine, as well. It's very funky, the variety of ways the body has to keep the ol' metabolic fires burning!

Fat is a great fuel, not just a social and medical problem.