When Aragorn uses the palantìr, he declares himself to Sauron and manages to learn something of the Enemy's plans. The communication both warns Aragorn of the danger to Minas Tirith and unsettles Sauron, causing the Enemy to attack prematurely. The declaration also initiates his passage through the Paths of the Dead, a traditional part of the mythic hero's journey. The visit to the underworld tests the hero's spirit before he can achieve victory (Frodo also passes through death on several occasions) as he demonstrates his courage in the face of death and the ability to lead men to face death themselves. Through the journey he transcends death and even commands the dead themselves, confirming that he is indeed the heir of Isildur and the True King.
The city of Minas Tirith is the remnant of a golden age, proud and majestic but on its way to ruin. Likewise, its ruler, Denethor, is a great man falling away into old age, whose pride will not allow him to accept the wisdom of Gandalf or even of his own son, and Faramir loves his father too dearly to challenge him. When Faramir slides into deadly illness and the armies of Sauron surround the city, Denethor's pride leads him into madness, and his lack of action could doom his beloved city — he chooses despair rather than seek help. In contrast, Théoden accepts the offer of help from Ghân-buri-Ghân and brings his forces to the field of battle just in time.
