Few of us own phone books today. Who needs'em with the internet. But if you have one or have access to them online through sites like Ancestry.com you'll be surprised at what you only thought you knew.
Sure you know where your grandparents lived or your cousin but do you know what their occupations were, if they were married or attending college.
For many years phone books provided a wealth of information.
They did this to better assist you with finding the correct persons. If your were looking for a friend or relative with a very common but you knew he was photographer, you'd be better able to find him by not only his name but by his trade.
Look at the journey of Paul & Vi Arpie & their kids stretching from the 1930's to 1980's. Multiple addresses & just about as many careers.
See their travels (mostly) through Ct at the bottom of the album: