It is also a good way to look up the level of any book they may be interested in reading to check that book's level.
Below is an example of the a listing you will find on the website. I have highlighted the book level- BL. A BL of 2.5 means that it is leveled at a second grade, fifth month level. That means that a student in the fifth month of second grade should be able to read this book. A book with a BL of 3.3 would mean a student in the third month of third grade should be able to read it. Obviously, this is not a completely exact science so you want your child to read within a range of a few months.
I hope this makes sense. Please let me know if you have any questions.