On a long steep downhill, if you use the truck & trailer brakes as the primary means of controlling speed, they will heat up (especially if you just ride them). Then when you have a situation where you need hard braking (reach a sharp curve or someone pulls out in front of you, etc...), there is the risk that the brakes overheat and lose braking power.
Instead, if you use primarily engine braking to keep the speed under control, you have fresh cool brakes on the truck & trailer that have their full capability available in those situations.
If the engine braking isn't enough, you want to do a short but fairly hard braking for a few seconds, then let the truck slowly gain speed before repeating. That will allow the brakes to cool off a bit between uses.
