Why Does Odysseus Go To The Underworld?

In Canto Eleven of the Odyssey, Circe advised Odysseus to go to the underworld before starting his journey to Ithaca to meet Tiresias to consult him and to speak with the spirits. Tiresias is a blind soothsayer from Thebes. Tiresias is in the realm of the dead (Hades) and can predict the future. Odysseus went to the kingdom of Hades to find out Tiresias' prediction about his future.
When Odysseus reached the underworld, he met Tiresias who told him that his journey to Ithaca will be very difficult. She told him to slaughter an animal and make the spirits drink from it to make them speak. In this underworld, he met several characters. He met Elpenor, one of his men that died in Circe’s house falling from the roof while he was drunk, he asked him to bury his body. He also met his mother who told him that she died of sorrow on him.
When Odysseus encounters Achilles in the underworld, Achilles states, “By god, I’d rather slave on earth for another man - some poor tenant farmer who scrapes to keep alive - than rule down here over all the breathless dead”. Achilles statement about the meaningless, grappling reality of the Underworld makes Odysseus realize Achilles contradiction to when he was alive. Achilles always wanted to die young with glory rather than old without any recognition or prestige. Without a doubt, Achilles learns an insight in death that his wrath blinded him to throughout everyday life: nothing matters except forever, not the unimportant contrasts of envious men, not the careless fierceness of a wronged companion.
In conclusion, having the guidance and assurance from the spirits of the Underworld gave Odysseus discernment in his decision making.