The U.S. Navy has found that it’s actually cheaper to use an F-18E aircraft for aerial refueling than to have other combat aircraft land and wait in line to refuel on the carrier. This landing and waiting consumes a lot of fuel. If the waiting aircraft turn off their engines (to save fuel) the aircraft has to undergo an hour of safety checks before it can turn on the engines again. Turns out it is easier and cheaper to send another F-18E aloft carrying six tons of fuel and use the usual in-flight refueling techniques to transfer fuel to aircraft that need it.
When possible, the navy prefers engines turned off during refueling but during training that emphasizes getting the maximum number of aircraft into combat, speed is essential. The U.S. Navy uses over two million tons of fuel a year for its aircraft. That’s about 600 million gallons to keep about 3,700 aircraft and helicopters flying.