Well casings are loaded up along the sides of the cab. The ends are placed in the pockets on the front fenders and rest of the pipe is laid out on the back deck through the opening at front corner of the deck itself. Once loaded you can't open the doors so you use the hatch. Well drillers had a yard right beside where I lived as a kid. They had one much like it back then. That's only the second one I've ever seen.
It's a center-cab truck for the oil fields. They can load well casings or drill stems, or whatever on either side of the cab, from the flatbed all the way to the headlights. That's why the fenders are diamond plate steel, and there's a sort of bucket on each one where it says "WELLS" and "PUMPS". When you have it all loaded up, you can't open the doors, so you go in and out the top hatch.
Well, I don't think I'd want a couple thousand pounds of pipe up that high if I could avoid it. Plus then you couldn't crane anything on or off the bed if there's pipe over it, and hand loading and unloading from 10 feet in the air has its drawbacks.
I used to have a neighbor that lived a few houses down from me that had an alcohol dragster and would fire that thing up every now and then. It was an awesome sight - and sound