I don't understand military and rank

If you watch Independence Day, you'll notice a side character who appears in most of the scenes in the ladder half of the film - Major Mitchell.

It's Adam Baldwin (Jane from Firefly) and apparently this guy does everything in the military - he's initially the Air Force liason to the President, then he's helping corale pilots, and then at the end he's driving the jeep to go pick up Ian Malcolm and Will Smith. Simple question - the hell is this guy's job? If he's the President's liason, why is he later driving the jeep? Doesn't he have other work to do? Maybe he could delegate driving to a subordinate officer. 

This is a general thing in movies - characters in organized, military or military-like organizations do a bunch of random tasks that they would likely just pass off to someone else. 

Take The Avengers. Why is Nick Fury the guy who is running SHIELD day to day, and then he's also the guy that needs to check up on the Tesseract. Wouldn't he give that to a specialist in that field of research? Later Black Widow (a special agent who job is to beat the hell out of people) is flying a plane on a routine trip back to SHIELD HQ. Wouldn't you want someone more specialized in flying to do that? I get that Black Widow is a bad ass and all, but I assume flying takes practice to maintain skill. So I wouldn't necessarily want her to be the pilot. I'd want a pilot, which I assume since SHIELD's HQ is flying a fucking air craft carrier, that they have a couple sitting around. 

See organizations have specialists for a reason, and leaders and important people need to do what they're good at and not waste time doing tasks beneath them. It's probably pretty insulting if you're boss is always coming over and taking the controls away from you. The upcoming jeep driver is thinking "Oh boy, here's my chance to drive the President. Moving up!" then Major Mitchell comes over and takes it away. Meanwhile, a hundred plus people are all waiting around for Major Mitchell to get back from driving so they can follow orders.