There’s lots of use cases in cities, too. As you said, most people live in cities, which means most construction work (both for businesses and for private citizens) will be in the city. While the companies that businesses contract with will usually have their own fleet of vehicles, many private citizens contract with smaller privately-owned companies. Many of them use their own vehicles for transportation. (I’m friends with the heads of three such companies in my area.)

Construction supplies (wood, pipes, etc.) like the kind many contractors get at Home Depot or Lowe’s is usually pretty long, so they would benefit from having a really long truck. Many of those companies usually need that supplies ASAP, too, so they get it themselves instead of having it delivered. Or, if they know they’ll need it, they order it ahead of time and pick it up themselves when it’s ready rather than trusting others to get it delivered on time. As others in this comment section have said, long objects like that create a risk of impalation. So the longer the truck, the better in that case. Not to mention they also need to store their tools and stuff in the truck alongside whatever supplies they’ve purchased.