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.