They light the forest on fire so they can "smoke" Ralph out of the forest, so that they could kill him when he got to the beach.
setting the forest on fire
They attempted shoving boulders into the undergrowth but it was to thick so it didn't work Then they set the forest on fire to smoke him out
He wants to set a fire, to try to get rescued
Jack and his hunters set the forest on fire because they want to find Ralph find kill him. The rescuer sees the smoke and investigates the island. Ralph rushes out of the fiery forest and he sees the Naval Officer.
Ralph's first decision as chief in "Lord of the Flies" is to organize the boys by assigning them tasks such as building shelters and maintaining the signal fire. He focuses on practical matters to ensure the group's survival on the island.
Jack's group tries to get Ralph by hunting him down with the intent to harm or kill him. They set the forest on fire to smoke him out and chase him relentlessly, demonstrating their descent into savagery and their willingness to resort to violence to eliminate their perceived threat.
The island is set on fire in "Lord of the Flies" when Jack and his tribe light the signal fire to smoke out Ralph and force him out of hiding. This act marks the climax of the novel as the chaotic events reach their peak.
Ralph feels that Jack's fire was too big and could have burned a lot of useful resources. Ironically, the fire will be more likely to signal help than the little fires that Ralph and the others built.
At the start of chapter twelve Jack lit a fire in an attempt to smoke Ralph out of his hiding place in the thicket close to castle rock. The fire simply got out of control, Jack did not intend to set fire to the entire island, including all the fruit trees and the pigs.
The hunters find Ralph by setting the forest on fire, thereby forcing him out of hiding. They use the fire to smoke him out of the thicket. This act symbolizes the destructive power of their savagery and their willingness to resort to extreme measures to capture Ralph.
The gamemakers made the forest fire to divert Katniss from the edge of the arena.