Probably, you are getting anonymous calls, or want to check who your significant other secretly talks to. If you rarely get calls that refuse to leave a message, you may not care about such calls.
Keep in mind that while they will give you the address, often apartment numbers may not be included in the listing, so if you will be sending something via snail mail to the person or business you are searching for, you'll want to make sure that you either know that they do not have an apartment or unit number, or that you have some other way of finding that out, such as by calling the phone number you find. If you have in your possession a residential phone number but no idea who the number belongs to or what address it relates to you can use a reverse lookup service to find out those details. Some free services however can provide you enough information to put the pieces together.
Sometime you will have to do a lot of searching and luck out. You can use reverse information sites when either of these are a problem. This can be very useful if you receive a call but you don't recognize the number on your telephone or mobile or cell phone.
Free directories don't cover cell phone numbers, it is that simple. As always your searches are guaranteed legal and confidential.