The bit between the truck and trailer is a drawbar. As mentioned, it is indeed mainly for manoeuvreability. In Australia, that combination is called a 'truck and dog', the dog being the rear trailer. The front axle(s) on the dog are steerable by the drawbar. The upshot of this combination is that you can tip the truck without having to disconnect the trailer, by 'folding' the combination up on itself, turning the whole lot into a U-shape.

As for load, these trucks are completely loaded, there is no waste. This is simply because the things that a tipper transports (bitumen, dirt, grain, gravel etc.) are very dense. Load limits, at least in Australia, are based on the amount of axles under the vehicle, so the only way to increase the load capacity of these trucks is to increase axle count. Example: https://www.aeitransport.com.au/biggest-load-to-be-moved-in-...

The image of the truck and dog you linked seems especially long. This might be a specialised setup for roadworks, where the driver can actually tip the load (in a controlled manner) through the drawbar and drive the dog over the top of it. The longer drawbar would leave room for the tipping body on the truck to raise completely, and also lower the angles experienced by the hitch if the driver offloads too much material between the truck and dog.

I'm late to this party, but I didn't see an answer that got to one of the big reasons for the large distance between a truck and trailer. There are laws about weight, specifically measurements called bridge and inner bridge.

My father and grandfather spent a lot of time reading the laws and charts every time they'd build out a new truck or trailer to try and maximize what they could haul. More axels allows for more weight, that seems obvious, but more distance is required between the truck and trailer as well.

It gets to be a really fun problem when one state wants a tongue/drawbar length of 28' and the neighboring state says that is too long and only wants 16'. What to do? Make the length adjustable and switch it over at the state line.

The last and biggest combo they ran was over 100' long, 9 axels, 34 tires and hauled 80,000 pounds. I don't remember the gross weight.

I would guess this has to do with maneuverability. It's very hard to back up a trailer when the drive vehicle wheelbase is long compared to the trailer. Adding a bunch of tongue length helps a lot.

Maneuverability. There's no articulation point between the cab and the dump. The gap between the dump and trailer allows for very tight turns.