A Gallic War Hound fills the defender role for a party of players. Perhaps your group only has four players, the player that plays a fighter will be missing the next few sessions, or no one in the group wanted to be a defender. The War Hound can take the missing role allowing your player party to be well rounded.
Maybe the hound will only be in play for a session or two, or maybe it will become a permanent NPC member of the group. As a dog, the hound is unlikely to have a key role in the story and the players can’t rely on it to handle role play or skill challenge problems. The treasure and magic item division issues don’t exist either.