Phonebook != Company Directory

Took me a bit to figure this out. In my IVR, I had an option to go straight to the "phone book" by pressing 9, but when someone did that, the list was empty. Found the phone book on the tools tab of admin page. Sure enough it was empty, and required me to add my own entries. To fix this, I just changed the IVR recording to have the caller press # to get the actual company directory that gets automatically updated when you add an extension with voice mail.

My question is, why have a phone book in the first place then if the directory already serves that purpose.? And how can one automatically update this phone book when a new extension is added if I decide to use it?