You see, unlike traditional novels, graphic novels have pictures that tell the majority of the story, with the words being there to aid the pictures as needed. Being a visual thinker, pictures are her natural way of processing the world. Pictures are how her memory best retains information. So a story told through pictures is ideal for her. She can process it quickly and easily. So she gets to enjoy the story rather than struggling through processing all those words that are in traditional novels.
Along the way, she is reading the words that go along with the pictures. This is building her sight vocabulary, her fluency, and more importantly, her confidence in reading!
I have a daughter who adores reading now! She consumes her graphic novels from the library and begs to go back for more each and every week.
(UPDATE: My daughter Hailey told me that if you read just the words, you'll only get part of the story, and if you just look at the pictures, you'll only get part of the story. She says you have to do both.)