My soapbox . . . Reading is comprehension. This should be the first and primary idea taught, demonstrated, and established in the child's formal reading program. A good read-aloud background will have already laid this foundation in the child's experience. Using predictable books will build upon that foundation.
Prediction is the core of reading.
Prediction and comprehension are intricately tied together. Their prior experiences with stories, print, and life in general are what enable them to process the text so efficiently in this manner. Reading predictable books helps a young reader to flow fluently through text because his expectations about what comes next are repeatedly confirmed. I've included a bibliography of books that you can use. The books are divided into levels of increasing difficulty.