The Bankers are one of the few neutral factions. Their primary purpose, as far as we've seen, is to maintain the storage vaults that are scattered throughout the Grand Canyon, which allow your items to stay secure.
Citizens that are unaligned with any of the 6 primary factions can associate themselves with the Bankers. No costume pieces or rewards have been shown as to what benefits there might be from this.
GenCon 2007 Interview with Lee Hammock