Paulie D’s comment maybe close to the truth. Although, Mark Henderson’s answer may be the “Hollywood” reason. The armed forces only move and fight with water, food, and rest. A lack of one of the above can be just as detrimental if not more so than a lack of ammunition. Any opportunity to eat, rest, and use the latrine facilities before an operation should be taken. You never know when the next opportunity will be. Letting your brain or your body run out of fuel when it is most needed can be detrimental.
Before airborne operations, more than half the unit would be either napping or eating portions of MREs. Soldiers waiting to don their gear would make sure they emptied their bladders and bowels. And, those that had already donned their gear would be sleeping either rocked back on their parachutes or back to back (chute to chute). Once in the aircraft, the majority of the jumpers would close their eyes and proceed to sleep. The ten minute warning was our wake up call.
To this day, if I am not the Pilot in Command, I fall asleep as soon as the engines start up. The drone is almost relaxing. On civilian AFF jumps, I sit as close to the pilot as possible so that he can tap me when we get to altitude.
If I am the Pilot in Command, I still make sure I hydrate, urinate, and have a light snack before I go out to the plane. And, I have a personal minimum number of hours of sleep in the previous 24 hours before flying.