Cooper used examples from classical literature and history in her essay because

Anna Julia Cooper was an American writer and activist in general, she grew around the "Reconstruction Era" in U.S.A., and was widely known for many of her speeches on civil rights and women's rights.

As a feminist in that time, being aware of the disparity between racial tensions, slavery, and the presence of women (especially black women) participating in the Reconstruction discourse, she wrote "A Voice from the South" around 1892, with the vision and purpose to help improving the living standards of slaves descendants, with "black women" playing a key role in that development.  

And in her writings she used examples of classical literature and history to show that women could do great things.

Explanation: