The main reason to apply low gears during a steep descent from a hill is to prevent your brakes from failing from overheating.

I can't believe some of you don't even know this. Did you not learn this in your driving schools?

When you drive down from Genting or Frasers or wherever, don't be a smart alec and go down with your gears disengaged and brakes only. There is a high chance that your brakes will fail from overheating. Please don't be an ignorant hero or you might end up ramming into other cars or even drive off a cliff.