are important because how well they do their job dictates whether or not a book can be found on the shelf. A library is dependent on its organization system (whatever it might be), so the process for getting returned books back to the right place on the shelf needs to be pretty close to perfect.
Pages are the first step in that process.
The second step is. Books get mis-shelved. Patrons pick up books and put them back in the wrong place. It happens. This is why it is necessary (however dreadful and tedious) to shelf read a library's collection from time to time.
Dodie Gaudet, on her blog Quick T.S., provides a nice Guide to Shelf Reading. It's kind of a recap of what is taught in library school, but distilled to the important parts, including suggestions and useful links.
Although I dread a massive shelf-reading project, we can always use one, and this might actually prompt me to begin.