This question refers to the story "Where Are You Going, Where Have You Been" by Joyce Carol Oates.
For the most part, Connie is a typical teenager of her time in this story. We learn that Connie is worried about the things that most teenagers worry about. She is concerned about being beautiful, and well-liked by boys. However, there is some individuality to Connie as well. We learn that she is a lot more vain than most people, and she takes enormous pleasure in her beauty. We also learn that Connie is dishonest, and she adopts different attitudes and personalities at home and elsewhere. The combination of these characteristics make Connie an individual, three-dimensional character.
The best answer here is the last one, that it allows the reader to sympathize with the monster. Before we hear his side, all we know is what Victor tells us and, by all accounts, the monster he created certainly lives up to his name. He murders his younger brother and is hideous. He chases Victor down and terrorizes him until he listens. This is all quite frightening, but reading the monster's perspective certainly changes things.
Through his narration, we discover that the monster only wanted to be loved and accepted like others that he sees. He is constantly rebuffed because of his appearance, the appearance that Victor gives him. Without this narration, we would continue to think the same way as Victor: that the monster is a terrible creature. But, because we are given a glimpse into the horrors he faced, we can't help but feel sorry for him because he is lonely and doing the best he can.
