1. Easier sign up process. Users don't need to know anything about S3 API keys or understand the implications of giving them to us.
  2. Security: you don't need to give us access to your generic S3 account. If someone compromises your regular AWS API Key they still can't get to your encrypted backup volumes and say... delete them.
  3. Cost transparency: TKLBAM related storage charges show up separately from your generic S3 storage.
FAQ: 
Amazon S3