Why did Odysseus go to the Underworld?

Why did Odysseus go to the Underworld?

Odysseus went to the underworld to seek advice from the ghost of Theban Teiresias about his return home. Teiresias warned Odysseus that he would face much suffering on his journey home and that he must restrain himself and his companions when they reach the Thrinacian island. He also warned Odysseus that if he harms the flocks of the sun, he and his men will face destruction. Teiresias also forewarned Odysseus of the destruction of his men and his ship, and of the trouble he will find in his house when he returns home. He advised Odysseus to take revenge on the suitors in his house and to carry a well-made oar with him on his journey. 

Here’s the key passage from Book XI, in which Odysseus meets and talks with Teiresias.

“Thus, then, did we sit and hold sad talk with one another, I on the one side of the trench with my sword held over the blood, and the ghost of my comrade saying all this to me from the other side. Then came the ghost of my dead mother Anticlea, daughter to Autolycus. I had left her alive when I set out for Troy and was moved to tears when I saw her, but even so, for all my sorrow I would not let her come near the blood till I had asked my questions of Teiresias.

“Then came also the ghost of Theban Teiresias, with his golden sceptre in his hand. He knew me and said, ‘Ulysses, noble son of Laertes, why, poor man, have you left the light of day and come down to visit the dead in this sad place? Stand back from the trench and withdraw your sword that I may drink of the blood and answer your questions truly.’”