The art of letter-writing is often the art of essay-writing in disguise.
Woolf while talking about women in literature made the germane quote above about women writers. Until the 1800's it was considered unseemly for women to be writers, but letter writing was considered to be acceptable. We begin to get a glimpse of the lives of women through women's eyes emerging in their letters. While many of the letters may have been somewhat bland, they did give some insight into the lives and concerns of women in the 1700's.
In fact some of these letters were lengthy and polemic and would nowadays be considered as essays..
Woolf's own letters also reveal much of life in late Victorian and early Georgian times.