Full size trucks generally have 8' beds with at least 48" between rear wheel wells; this allows one to carry the stereotypical sheet of plywood laying flat in the bed.
Smaller trucks can carry plywood flat if the load is raised above the wheel well and the tailgate is opened. Slots (detents, cutouts, indentations, whatever) in the bed accept lateral 2x4 or 2x6 boards. The height of the top of the 2x6 is even with the wheel well. This means with the tailgate open the plywood is supported fore, aft, and on both sides.
On my 2003 Ranger the cross-pieces will be 57-58" long. 57" will be able to drop in from the top and 58" will have to be wiggled in from the side. There are also creases in the wheel well that fit the edges of the plywood to keep it from shifting so much.
So let's subvert this plywood carrying functionality for our own (camping, sleeping platform) purposes. Go ahead and put up the cross-pieces and throw the plywood (5/8"? 3/4"?) on there.
Cut the length down to 6' so the tailgate will close.
Cover it with something (outdoor carpet?) to keep the splinters under control.
Toss on a used topper off craigslist and you have a serviceable camping shell. Good for frugal traveling, emergency naps, zombie apocalypse, evacuations, "urban camping", whatever. Woo hoo!
why a shell and not something else
a shell can be cheap on the used market, particularly if ugly
a shell can be removed without much fuss
a shell is reasonably rainproof and can be locked, unlike an open bed
a shell can still accommodate reasonably sized cargo, unlike a tonneau.
a shell doesn't need setup like a pop-up bed tent, though I think those are a good idea for many people.
a shell is dorky but more common and discreet than a cabover
a shell is always there, unlike a pop-up trailer
my father built a sleeping platform in a pickup bed w/shell and I thought it was great when I was 12.
why a full-width deck rather than a bed
because I am married
because I have no carpentry skills. You should see the chicken coop I built -- good thing chickens don't care.