Originally Posted by Jono
.."Brakes to slow, gears to go"....
Tell that to someone coming down the Alps with a 44 ton load......
The advantage of using the engine to dissipate energy is this immediate ejection of energy. Hot gases are ejected from the vehicle very quickly and the gases also transfer much of their heat directly to engine parts. In addition, friction produced within the engine system also adds heat to the engine parts.
This engine heat is taken away by the engine's integrated cooling system: usually a liquid circulation system and a radiator. Disc or drum brakes have no such energy dissipation mechanisms. They must rely on air flow to remove heat and they use their mass to retain heat without producing temperatures that would deform and damage the brakes.
Placing a vehicle in a low gear causes the engine to have more leverage (mechanical advantage) on the road and the road to have less leverage on the engine. This is what allows cars to slow down using their relatively flimsy engine parts. The engine maintains a high rotational speed to dissipate a lot of power without forcing too much strain on the engine.