I've seen those something similar years ago. I think they were a way to extend the cab before the stretch and crew cabs became the norm. Not very useful if you needed to carry other cargo.
It is what is needed for commercial pickup truck drivers who haul when driving time exceed federal hours of service limits to use for Legal sleeping in your vehicle or else you have to stay in some no tel motel or hotel. Sleeping in the rear seat of a crew cab doesn't qualify by DOT laws as a sleeper.