What was Garvey reasons for agreeing that isolation was good

Banishment isn't punishment. Garvey explains that the Tlingit Indians have used this method of self-discovery for hundreds of years. The banishment would be a "vision quest", an "extended time alone to face himself and to face the angry spirits inside of him." On this island Cole would be free of peer pressure, drugs, alcohol and all the vices of his old life.