Probably increases vocabulary and reading level. Parents seem to be more motivated in getting their children to read AR books. Increases circulation of library books. Encourages friendly academic competitiveness among students and teachers. Gets kids to read books that are appropriate for them.
Students get immediate feedback.
Teacher doesn't have to spend time tracking their top readers as AR does it for them. Gives them more time to work with lower readers. All books are leveled by the same formula. Teachers love the proof of having the AR record to present to parents at conference time. Teachers and students know their zone of proximal development and books can be selected accordingly.