Thomas Builds-the-Fire is Victor's guide and catalyst in this story. Victor has lost touch with his father and his Native American roots and heritage. Thomas is the one who helps him reconnect and, in a sense, be reborn. Thomas repeats these stories because he is deeply connected with that heritage, with memories, and with finding signs and symbols in things. Victor is struggling to be more modern, so he has no time for such things.
During the course of their journey, Thomas tells Victor a story about a dream he had. The dream told Thomas to go to Spokane and wait for a sign. Victor's dad showed up, took Thomas out to eat, and drove him home. Thomas interpreted this as the sign and told Victor, "Your dad was my vision. Take care of each other is what my dreams were saying. Take care of each other." Victor's father took care of Thomas that day, and Thomas takes care of Victor on this journey. Victor seems to accept this connection: here, we have one of Thomas's stories putting things together.
The title is "This is What it Means to Say Phoenix, Arizona." In Greek mythology, the phoenix is associated with the sun. It could burn and be reborn of its own ashes. This myth parallels the overall story of Victor's rebirth, or reconnecting with his father and Thomas. Remember that Thomas is the guide for Victor. They are traveling to Phoenix: a city with the same name of the mythological bird. They retrieve Victor's father's ashes. And Thomas Builds-the-Fire helps Victor start this whole process for Victor: fire, ashes, and rebirth (just like the phoenix). It is Thomas's story about Spokane that really gets through to Victor. His stories and friendship make this process possible.
Why did Thomas have to tell stories? From the context of this story, we can tell that it is because he wanted people to take care of each other—and to do that, they must listen to each other.