The fire is the boys' only hope of rescue from the island. When the boys go hunting and instead of minding the fire as Ralph instructed them to do, a ship passes by the island. Had the fire been lit, the people on the ship may have seen the signal and rescued the boys. This is why Ralph is angry; the other boys' disobediance ruined his chance of escaping the island.
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Because it isn't producing enough smoke to make it noticeable to any ships or airplanes which might pass the island.
When they let the fire go out, Ralph gets upset because a ship passes by and they can't see them because there is no fire. The smoke signal is their only way to get be seen and get rescued.
Ralph spoke. "You let the fire go out."
A ship is passing along the horizon. However the hunters let the fire for when it was their job to keep the fire going. Therefore the ship did not know they were there and by the time they got the fire going again it was too late. Ralph becomes furious because they could have had a chance to be rescued.
In Chapter 10, Piggy suggests that they let the fire go out at night and relight it every morning because he concludes that the smoke will not be visible at night. At this point, Ralph is frustrated that Jack has started his own tribe. Ralph is also discouraged that only he and Piggy seem intent on keeping the fire going. Upon hearing Piggy's suggestion, Ralph agrees to let the fire go out. However, Ralph thinks it would be ideal to have the fire going all the time. The fire also gives Ralph a sense of security, especially at night: Ralph stood up, feeling curiously defenseless with the darkness pressing in.
They decide to let the fire go out at night and light it again in the morning
To watch over the young kids.