described as being a source of light, and in this endless darkness, Wiesel’s outlook on life and religion changes irrevocably:

Never shall I forget that night, the first night in the camp, that turned my life into one long night, seven times sealed.

As this quote explains, Wiesel feels that his life has been transformed from days and nights into one everlasting night. In Jewish tradition, the number seven creates a symbol of completion, and here Wiesel is telling us that he feels as though the night into which he has been plunged will never end.

A second reason for the title of Night relates to a specific event in the novel, in which Wiesel sees the flames of a blazing inferno contrasted against the darkness of the night. Small children and babies were being thrown into this fire, and Wiesel remarks that it is little wonder that since this experience, sleep tends to evade him. He even pinches himself to make sure that he is awake, wishing to find that he was asleep and that this entire “night” had been nothing more than a nightmare.

A writer's purpose can usually be found in the themes that he develops in his work. In Elie Wiesel's Night, there are several themes that run throughout the work. 

When Wiesel writes about the cruelty of the Nazi soldiers in  the concentration camps, he is developing the theme of man's inhumanity to man. This becomes poignantly evident when we see changes in Elie himself. At one point he tells the reader that because of his hunger and deprivation, he had become nothing more than "a stomach." 

However, Wiesel also develops a counter-theme of kindness under severe conditions. Despite the suffering and ever-present threat of death, there are still moments when people are kind and giving. At one point, a young violinist named Juliek, who somehow managed to hang on to his violin in the midst of forced marches, plays beautifully for the exhausted and dying prisoners, only to die himself soon after. Elie's father continues to give Elie his own rations as he faces starvation himself. Sometimes prisoners risk their own safety to give comfort to those who are suffering. 

Wiesel's main purpose is no doubt to ensure that the world knows what happened in the camps. His own characters persistently denied their fate early in the book, refusing to believe the things they had heard about the Nazis' plans for them, refusing to believe that such terrible things could happen to them in the twentieth century. Wiesel wants to make sure that the rest of the world doesn't make that mistake.

Wiesel's purpose is to give witness to the atrocities of the Holocaust, to speak for the millions who died in the camps. By documenting his experiences, Wiesel hopes to warn future readers about how far hatred and prejudice can go in inflicting death and violence.

Wiesel often said that to remain silent was one of the worst things he could have done after all he lost and all he saw other Jews lose. During his Nobel Peace Prize acceptance speech, Wiesel said, "I have tried to keep memory alive. . . . I have tried to fight those who would forget. Because if we forget, we are guilty; we are accomplices."

That is the main purpose of Night: to make sure that in the future, as living witnesses of the Holocaust die out and the event fades from living memory, that future generations will not forget what happened and that people will endeavor to make sure such atrocities never happen again.

Eli Wiesel tells us in an interview that he wrote Night partly to speak for the dead, and for the survivors who could not speak of the atrocities. He told the story of his survival experience with a sharp honesty that would help those of the future also have an accurate representation.

He witnessed and experienced some of the harshest experiences possible for a child of his age, and although the writing is from the memories of an adult author, the voice is still strikingly innocent. In this way, Wiesel communicates the memories of those who shared his experience.

His observations, descriptions of the struggles of those around him, and description of the actions of Nazi soldiers and guards all work together to document historic events, preserve the memories of himself and those around him, and serve as a warning to future generations about the harsh reality of genocide.

He did not seek to be seen as a hero, or martyr of any sort. Rather, he wrote to preserve the truth, speak for a generation, and give the future something to consider.

This question can be answered in several ways.  One reason why "Night" was written was to speak to the world about the ordeal of the Holocaust from a survivor's point of view.  I am not sure any History text can effectively relay the horror, atrocity, and terror of The Holocaust any better than Wiesel's work.  One need only read about how he was whipped or the closing lines where he stares in the mirror only to see a corpse staring back to understand the true impact of The Holocaust.  Another reason it was written might have been for Wiesel's own voice to be acknowledged.  Anyone who has endured an trauma on the level of Wiesel's must speak out, must take back what was taken, and use their voice in articulating their narrative.  I am certain that Wiesel must have had a psychological compulsion to pen the book of his experiences.  I think that another reason why "Night" was written was to alert the world of the danger in remaining silent.  While Wiesel speaks very clearly to the sadistic cruelty of the Nazis, he is also quite forceful in speaking out against the Jewish individuals who did not say anything when they recognized atrocities were happening.  When Moshe the Beadle is taken, many dismiss him as insane, and few, if anyone, speak out about his rights being violated.  This understanding, that silence only benefits the perpetrators of cruelty, is an extremely important element in Wiesel's "Night."  It is a theme that Wiesel himself speaks to quite eloquently throughout his career.  In a speech delivered at The White House entitled "The Perils of Indifference," Wiesel summarizes it:  "Indifference is a sin."  These reasons are only the beginning of why "Night" was written.

This book is called "Night" because we associate night with darkness and we associate darkness with evil.  The book is a story about a time when evil took over much of Europe.  Therefore, the title makes sense.

The Holocaust (which is, of course, what the book is about) was in my opinion the most clearly evil thing that has been done in the life time of anyone living today.  This means that is appropriate to talk about it as a night, a time of darkness.

In my opinion, the book is also called "Night" because of the darkness experienced within the souls of those who went through it.  The book explores, among other things, how the characters think about God and how they stay (or do not stay) sane through their ordeals.  You could see these struggles as "nights," times of darkness for the victim's own souls and sanity.