The main purpose of the use of repetition in the lines is the one we see in the statement below:
It forces the audience to examine their ideas about African American women.
By asking repeatedly "ain't I a woman?", the speaker forces the audience to reconsider the way they see African American women, since back in the days of slavery they were often regarded as merchandise and nothing else. By comparing herself to a man, the speaker means that since she can do all a man is able to, she is indeed of great value, and must be seen as a human being whose gender is female. In other words, she wants to be acknowledged for what she is in every sense. That's why what she says has the objective of making the audience ponder about how they see African American women.