The more familiar a reader is with the author's life the more they tend to replace the narrator or main character of the novel with the author. For example, instead of the fictional character Jane Eyre being desperate for love and approval, the reader might see Charlotte Bronte as being desperate for love and approval.
More info about the author's life can also put the novel into context, especially if the novel is not modern. Readers can draw upon their knowledge about that author's life in order to better understand actions, dialogues, and themes in the novel from the perspective of a person who lived in that era.
Being familiar with an author's life can leave the reader with a deeper understanding of the novel.
Hope that helped!