Why is the troll a problem for the three billy goats gruff?

The troll is a problem for the three billy goats gruff because he lives under the bridge they must cross to reach the lush green pasture on the other side. The troll threatens to eat them if they try to cross, instilling fear and creating a barrier to their journey. Each billy goat must outsmart the troll to safely reach their destination, showcasing themes of bravery and cleverness. Ultimately, the troll represents an obstacle that must be overcome for the billy goats to achieve their goal.