Why does Aeneas visit the underworld in book 6 of the Aeneid?

The correct answer is: to see Anchises, his deceased father.

Explanation:

After receiving the Cumaean Sibyl's prophecies that the Trojans were still to endure great hardships, Aeneas begs her to let him descend to the underworld to see his beloved father, Anchises, once last time.

Aeneas needs his father's loving and encouraging words to face his destiny and finally become the founder of a new race: the romans.