If a biography is authorized, then you can interview the subject, and in some cases those interviews provide a lot of the material. Walter Isaacson's biography of Steve Jobs was like this, for example.
- Access to records, correspondence, and journals in the subject's possession
- explicit endorsement from the subject encourages their family, friends and colleagues to work with the author.
- Opportunities for the subject themselves to fact-check and contextualize the research.
That must be balanced against the fact that authorization also often provides the subject an opportunity to review the content, giving them a hand in crafting the narrative and/or suppressing unflattering details.