How could such a bed be useful? They're incredibly useful!
They're useful to load things quickly. Awkward objects can be placed in the beds without having to carefully maneuver around a vehicle interior and all its obstructions. I do a lot of woodworking. Before we got a pickup, I would transport 2x4s and other lumber inside a Toyota Corolla. I've discovered it is actually possible to fit several dozen 8 foot long 2x4s inside a Corolla. But the pickup truck is such a better tool for the job. Plus you're not moving a full sized sheet of plywood in a sedan.
They're useful fit to large objects that would never fit inside a vehicle. If properly strapped down, a pickup can transport an object much taller than the roof of the truck.
They're useful to move really dirty things you wouldn't want in your car interior. Imagine you want to fill some garden beds with mulch or compost from a garden supplier. You could get it in small bags, but that would be expensive. It's cheaper to buy in bulk. You could have them deliver it, but that would be more expensive still. If you need to move soil, compost, rock, or anything else dirty in bulk, a pickup is the way to do it. You can have the garden center fill your truck bed with a back hoe. Just dump it right into the bed. Then shovel it out when you get home. To clean out the bed, all you have to do is hose it off.
Now, this is pickups in general. We have a mid-2000s Ford Ranger, which is much smaller than the trucks all the big American manufacturers are selling today. A small pickup is a great utility tool. A giant one is a pointless penis extender.
There is not an auto manufacturer on this Earth that isn't heavily subsidized by its government. The same industry, skills, and infrastructure that make it possible to build consumer vehicles also allow you to build military vehicles. In wartime, a government can come to an auto manufacturer and tell them, "you're done making consumer pickups. You're making army trucks now!" And domestic industry cannot be blockaded or embargoed.
And that's not an industry you can just spin up overnight. If your country doesn't have an auto industry, and you would like one? Even if you have an unlimited budget, it would still take you decades to get to the point of competitiveness. Countries have a lot of incentive to subsidize their domestic auto producers as a means of ensuring the country retains the ability to make its own military vehicles.