Margaret Cavendish had her portrait engraved on her works to promote herself as a serious author. She was a woman in a male-dominated field, and she wanted to show that she was a capable writer. She also wanted to attract more attention to her work, and she thought that having her portrait on it would help to do that.
The Marvelous Mind of Margaret Cavendish:
An Engaging English Quiz on the Duchess of Newcastle-upon-Tyne