The axle just behind the cab in your illustration is called a pusher axle and the axle mounted on the rear of the trailer is called the tag axle.By law trucks are allowed to carry a limited amount of weight on each axle.A dead axle allows the truck to legally carry a heavier load.
The amount of weight that a truck can have per axle is decided by the department of transportation in each U.S. state.When you load a dump truck with gravel, the weight can easily exceed the per-axle limit, so the extra wheels/axles are lowered to make the truck legal.
They are raised at other times to improve gas mileage and eliminate wear on the tires.The axle is lowered to increase the weight capacity, or to distribute the weight of the cargo over more wheels, for example to cross a weight restricted bridge. When not needed, the axle is lifted off the ground to save wear on the tires and axle and to increase traction in the remaining wheels.
Lifting an axle also alleviates lateral scrubbing of the additional axle in very tight turns, allowing the vehicle to turn more readily. A dead axle, also called lazy axle, is not part of the drivetrain but is instead free-rotating.The rear axle of a front-wheel drive car may be considered a dead axle.