How is Circe a valuable ally to Odysseus in the beginning of Book 10?
In Homer's Odyssey, the title character encounters a number of powerful women. One of these is Circe, who lives on the island of Aeaea. In Book 10, Homer describes her as "a fateful goddess with a human voice". Early in Book 10, she turns several of Odysseus' men into pigs, but later in the book, after Odysseus demonstrates that he is immune to her magic potion, she treats him quite nicely. Additionally, she restores his comrades to their human form and even adds some improvements: "...they became men again, younger and handsomer and taller by far than they were before."
Circe takes care of Odysseus and his crew for an entire year, before finally his men persuade him that it is time to leave. Before he departs, though, Circe tells Odysseus that he must travel to the land of the dead to consult the spirit of the Theban prophet Teiresias. She goes on to explain how Odysseus can travel to this land and the ritual he must perform to conjure up the dead. Furthermore, she even provides Odysseus with the offerings he will need for this ritual:
Circe went on ahead of us, and tethered a ram and a black ewe by the black ship. She had easily slipped by us: who can observe a goddess passing to and fro, if she wishes otherwise? (A.S. Kline translation)