During long distance highway driving, foot/ankle muscles get tired out and sore holding the gas pedal constantly. Also, some people are very bad at maintaining

During long distance highway driving, foot/ankle muscles get tired out and sore holding the gas pedal constantly. Also, some people are very bad at maintaining a constant speed, frequently accelerating and then releasing the pedal, so their speed looks like a sine wave, which absolutely destroys fuel economy. For those people, cruise control improves efficiency.