I think the more important reasons are to prevent the groceries from sliding around in the bed and to protect them from the sun and precipitation.
Still see this in the ones with tonneau covers as if they intended to put cargo in the bed
But who knows maybe it’s already full of stuff under there
Dang, if only someone made an everyday car with a soft weather proof enclosed cargo area, possibly you could have the tailgate open up and away as well so it's not in the way. I would think Americans would love to have something more practical than a work vehicle