Having a VAX Passbook is your vaccination record showing the history of all the vaccinations you’ve had. It helps keep your vaccine records up to date and these records are increasingly likely to be required for certain jobs, travel abroad, or school registration.
Plus, there is no national organization that maintains vaccination records. Not even at the CDC. The only records that exist are the ones you or your parents were given when your vaccines were administered and perhaps in clinic records where the vaccines were given