Music makes children's books a lot more fun to read. But that's not all it does. It has been proven through the study of neuroscience that music stimulates the brain. A stimulated brain learns better. Memorizes more. Recalls information. Music unlocks your child's ability to learn to read.
First step children's books are narrated, which is important--the child discovers the correlation between the word they hear and the one they see--this is how they learn to read. The music engages them and stimulates their mind, enabling them to remember they words they have just seen/heard.