If your job requires you to travel throughout the country, you cannot walk hundreds of miles between cities. If you need to get quickly from LA to Boston because your mother has suddenly taken ill, hitch-hiking your way wouldn't be much of an option.
In most cases, the only practical way to get around is by car, plane, or train, and each method requires papers. (Yes, you can mostly get around in your car without having to show anybody papers, but how long before cars are tracked through methods electronic or otherwise?)
So for most people who lead normal lives, you really do need papers to travel.