Children feel comforted by the familiar. They like familiar foods, routines, toys, and even books.This familiarity in a special book though builds a child's skills needed for reading. A parent models reading fluency, speed, and expression while reading and the more this is modeled, especially with the same book, the more a child tries to emulate this. Reading comprehension also grows with a familiar story, as a child practices anticipating what will come next and has discussions with a parent about what happened in the story. And finally, vocabulary is grown, as a child needs to hear a new word repeated a number of times before committing the word to memory.
So, as difficult as it can be to read "Green Eggs and Ham" for the 300th time, just remember that your child benefits from each reading...and re-reading...and re-reading!