what is the goal of banishment according to the First Nation people.

From the text:

Garvey shook his head. “Banishment isn’t a sentence. It’s simply a time for Cole to walk his talk. We tell him to give more than lip service to the idea of change, but what chance does he have to prove anything if he’s locked up? Sentencing would be delayed until the end of Cole’s banishment. Then this same Circle could reevaluate him and decide whether he has walked his talk, and whether a sentence is still necessary.”