Couldn’t you just put the truck in neutral and use the regular brakes?
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When you use regular brakes, you are actually using the friction of the brake pads to slow the vehicle. This heats up the brake pads (and discs/drums) as well as wearing them out. The wear is small, but it adds up the more you use them. After you use the brakes enough, you will have to replace the pads. After a few times of replacing the pads, you will have actually worn enough of the steel brake discs/drums away that they will have to be replaced!
In addition to the wear, the heat can be significant. Heat into brakes can make them work less well, and can cause additional wear and even failure of brakes.
So anything you can do to use the brakes less is useful. It does not matter as much for small cars (even American SUVs), but it matters with large, heavily loaded trucks. One way to slow a car (or smaller vehicle) is to just coast. The friction and air resistance, even without the brakes, will slow the car down. Fully loaded trucks have so much weight (and momentum) that coasting will take a long time.
Instead of coasting or brakes, it is possible to use the engine to help slow down.
The engine makes power to accelerate or maintain speed, but it takes some of that energy just to run the engine itself. It turns out that you can increase the amount of energy it takes to run the engine, just by having the wheels turn the engine, instead of the other way around. If you do not give the engine any fuel, the wheels (and the momentum of the vehicle) are having to force the engine to spin.
Now, it is important to remember that an engine has to move air through it. It takes in fresh air, adds fuel, then burns that air to push on a piston (which creates the energy to turn the engine). After that, it has to get rid of the “burned” air through the exhaust. If you restrict the air coming into the engine, and you restrict the air going out of the engine, it makes it harder for the engine to turn. If you do not add any fuel, then the engine does not create any power, and so all of that air restriction means that it takes a lot of energy to turn the engine.
The end result is that the engine does not burn any fuel, and it helps slow the truck down without using the brakes. This does not do any harm to the engine, since the engine is just doing what it would normally do (minus burning fuel). Often, a heavy vehicle can go down a long declined road down a mountain, and only use the engine brake, or use the regular brake in short amounts. This is better for the brakes, and since the brakes will not heat up, it means that the brakes are in better shape for emergency use or intermittent use down the hill.
Even just slowing down from cruising speed is better done with the engine brake or engine brakes plus regular brakes, instead of just the regular brakes.
Note that engines do not create energy, heat and friction are more complex, and may other liberties were taken in the explanation above, due to ELI5 reasons.
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