Narrative Transvestism

Many of the earliest canonical novels--including Defoe's Moll Flanders and Roxana and Richardson's Pamela and Clarissa --were written by men who assumed the first-person narrative voice of women. What does it mean for a man to write his "autobiography" as if he were a woman? What did early novelists have to gain from it, in a period when woman's realm was devalued and woman's voice rarely heard in public?