The best biographies of all time are those that have left echoes in our collective consciousness.
Precisely because of the legacy left by the subjects of those biographies — and the biographers’ skill in sharing that legacy — we can climb to heights made accessible by the giants who came before us.
The perspective of someone not in the subject’s head may feel less reliable or at least less intimate.
But it’s also more likely to be objective and dispassionate.
The biographer might share things the subject would not — either from modesty, shame, or something else.