They light a torch, and the murderers set upon them. The murderers kill Banquo, who dies urging his son to flee and to avenge his death. One of the murderers extinguishes the torch, and in the darkness Fleance escapes. The murderers leave with Banquo's body to find Macbeth and tell him what has happened.
Answer:
It was important, because without it he would've been dead. He needed it in order to trick cyclops into not eating him right away.
Because cyclops keep to themselves and are uncivilized.