The Rez

The rez is important to Arnold and his family in many ways. First of all, it represents community and the bond between the tribe. If there was no common land, there would be no direct link between all of them. Although there is a spiritual connection, they would have no reason to stay together without the rez. The reason they all know each other is because of the rez.
The rez also represents obligation. No one wants to leave because they feel like they can't, or that they don't deserve better. This leads to poverty and alcoholism, which creates a cycle. Everyone feels obligated to stay together since they all feel unable to leave. Arnold breaks this cycle by going to Reardan, but even he says himself that everyone from the rez will be mad at him for leaving.