Why did the author include Matt and his dog in the novel?

Matt's family lives in squalor in the Fen, and at one point during the novel, his mother says that she is glad to be rid of him. Even Thomas proclaims that Matt has no worthwhile skills, though Kira is able to see otherwise. She recognizes the good in Matt's heart, as well as his fierce loyalty. In many ways, Matt's dog is an extension of Matt.