Cell phone directories are used by people for a number of reasons whether they want to find somebody calling them constantly and they are feeling harassed by their behavior or in order to verify a phone number coming from a company offering a job to an applicant or it can be used by people to find if a number is legit as far as the company or hospital is concerned. For that reason, often governments have released cell phone directories containing the details about the people registered under the name and you can even read the review on cell phone look up directories giving an idea about what makes the good and accurate phone directory to use;
In case you lose your contact list:
You can find the owners or names of the people in your friend list by looking at your call list in case you accidently delete the contacts in your friend list and there are so many unknown numbers now lingering in your phone without names and leaving you wondering who these people are as you cannot simply call at these numbers to annoy each and everybody and then determine who they are.
Knowing the whereabouts of your lost friends:
It is pretty common that when friends split up after school they are no longer in contact with each other because everybody gets busy in their lives and forget to stay in touch but it is very likely that after many years of your life, you feel like getting back together and enjoy some time and relive the school memories and thus a cell phone directory often has the option to look up for a person by their name and by entering the name you can get information about that person such as their address and this helps the long lost friends get together.
Knowing information about the deceased persons:
Sometimes the people who die in unfortunate accidents or who are too old to live but might be of interest to you in order to pay your regard to their families and pay your visits to the grave and many cell phone directories allow you the chance to look up names and give certain amount of information in case they are not alive, it might contain some information about this too.