They're only used for fuel economy. They have less rolling resistance somehow.
Do you remember the "Aqua-tread" tires that were supposed to be impossible to hydroplane with in the rain? They have a deep groove in the center so they channel water out of the way. You get the same effect with a set of duals. A heavy truck needs "X" amount of tire width to distribute the weight. Duals accomplish the weight carrying capability and by having a gap between the two tires, they don't hydroplane.
A single tire with twice the tread width as a single conventional tire acheives the same weight loading but they lose that water channeling ability. So they're more likely to have traction issues in bad weather.
As for the red "thing" in the pics above, I think it desperately needs some big tires .. it just looks stupid with a full size Peterbilt cab and hood but car tires. If it's 2WD and used on the street, tall skinny commercial tires, conventional truck tires could be used. If it's 4WD, I'd just go with big mudders.