What was the second fire for in "To Build a Fire"?
The man builds the second fire because he has broken through some ice and soaked his feet and legs all the way up to his knees. His main purpose is to keep his feet from freezing, because that would amount to a death sentence. He would be unable to walk and would eventually run out of the limited fuel available for his fire. He has built the fire under a spruce tree because the only fuel available is twigs he breaks off the tree. But breaking off the tough twigs has caused the tree to shake slightly. It is heavily laden with snow. Some snow falls off the higher branches onto lower branches and starts snow cascading down like an avalanche. The man has taken off his moccasins to dry them over the fire. He and his fire are both buried under falling snow. Now his hands are so numb that he is unable to gather more wood or to handle his remaining matches in order to try to start another fire. He is doomed to freeze to death, and his dog abandons him.