Since the beginning, there have always been two pilots up on the flight deck. Had it not been for the Wright Brothers, maybe we'd only have one seat up there. It's a common misconception that the First Officer (commonly referred to as the copilot in the movies or 'gear monkey' in real life) isn't really a pilot. This is false. They are just as qualified as the Captain. The real question though is... are they essential?
This takes us back to the Wright Brothers.
Pilots are narcissists who need an audience. We need someone to laugh at our jokes and make us feel important. We need someone to entertain us when we get tired of monitoring the autopilot. We also need someone to humbly do the dirty work so we can keep our hands clean.
This need for validation is what encouraged the Wright Brothers to take to the skies in the first place. That and sibling rivalry. The day Orville beat Wilbur in a bike race is the day Wilbur said, "Oh yeah! I'm gonna put wings on a bike! Lets see who's laughing then, Bro!"
And the race to the air began. While the endeavor initially was a game of one-upmanship, soon the master/sidekick team was traveling the world demonstrating their magical flying machine. The modern First Officer was born. Orville out getting the Flyer ready, Wilbur winking to the ladies in the audience, drinking his coffee and passing out rectangular wings to the kids.
Yes, of course the first aircraft only had one seat but as flight durations got longer than a single hill the sole Pilot in Command grew tired of hearing himself tell the same corny jokes out loud and demanded a second seat be installed for comic relief. It was only natural to bring along the guy that new almost as much about the airplane since he was out preflighting and making sure the Captains name was painted crisply and legibly under his window. Oh, and if the paint was chipped... There would be hell to pay.
So, a second seat was mounted but had no actual controls. It was there simply to hold a warm body. And to balance the airplane, of course. (As planes got longer, flight attendants were also added for this reason.)
In time, as aircraft reliability increased, naturally the First Officers abilities improved as the bar for excellence was lowered. A natural medium was found balancing the skills of the cockpit crew. A give and take relationship has developed where the copilot is there to ensure the Captains ego isn't damaged by laughing at his jokes and ensuring the flight is operated safely so no paperwork has to be filled out.
They are essential flight deck officers who may be replaced by Siri now that we can use our portable electronic devices through all phases of flight.
I'm just not sure an Iphone can run the checklist so I can concentrate on what I will say to Letterman or Anderson Cooper in the event of an unlikely water landing.