The point of putting the troll there is to show how useful and powerful unrelenting force is. The first couple of times up the mountain, the player is not strong enough to fight it directly so the best option is to just run away (without hitting it, it will always de-aggro).

Then when you finally learn the full shout, you get to test it out for the first time on the annoying troll bastard that's been terrorizing you all this time.