This contribution aims at presenting the reason why late antique biographers wrote the lives of Moses and Pythagoras. To show the use of such lives by late antique biographers such as Philo, Gregory of Nyssa, Porphyry or Iamblichus, the first part of this contribution looks at the justification of such a biography by the author himself, through the title, the introduction and conclusion of his text, as well as its editorial context. The second part moves away from the text itself to study the social, professional and doctrinal position of the biographer. The study of the author's "posture" thus makes it possible to understand the common function of such biographies: presenting a life model and invite the reader to imitate this model.