Being able to locate a mobile phone has many uses. Usually a mobile phone is with it's owner, so locating the phone usually means locating it's owner. This can very often be a matter of personal safety. If you are kidnapped, then the police may be able to track your phone, and so you. If you are lost, then you can call the emergency services, who can track your phones signal to find your location. On the other hand, if you lost your phone, there are applications you can use to find your phone from another phone. The simplest and cheapest method is called triangulation. For this to be effective, you need to be within range of at least three mobile phone masts. The masts ping or send signals to your phone. By analysing the time between a signal being sent and received back, your distance from a phone mast can be estimated.
If signals are sent and received from at least three phone masts, then your location can be narrowed down, in principle, to unlimited accuracy, and in practice to about 300 m.