GPS (Global Positioning System) locates a device on the ground using satellites operated by the United States. A receiver picks up signals from roughly 30 satellites and, by calculating its distance from four or more of them, fixes latitude, longitude, and altitude to within a few meters. Indoors, or hemmed in by tall buildings, the satellite signals struggle to get through, so accuracy drops and getting a fix can take longer.
For telephony, the most important role of GPS is location notification on emergency calls. When a mobile phone dials 110 (police), 118 (Japan Coast Guard), or 119 (fire and ambulance), the location is sent along with the phone number to the answering position without the caller doing anything special. What gets sent is not always a GPS fix. On handsets that support GPS-based location notification, and where GPS is usable, the GPS fix is sent; otherwise the location is calculated from the base station site and sent instead. This location notification function has been mandatory for telecommunications carriers since April 2007.
That said, the Ministry of Internal Affairs and Communications notes that GPS reception and radio conditions can prevent an accurate location from being reported, and advises callers to also state their current location and nearby landmarks out loud. Automatic notification is not a reason to skip being ready to describe where you are.
GPS also earns its keep when a smartphone is lost or stolen. Apple's "Find My" and Android's "Find My Device" plot the position the handset measured on a map, and can lock the device or wipe its data remotely. The point people miss is that whether the handset can measure its position and whether that position reaches the map in your hand are two separate questions. With the power off, or with communication cut as in airplane mode, the device itself cannot send out its position. Some supported iPhones and other devices, however, can still be located for a while after power-off through nearby devices (such as Apple's Find My network). Practically, that means checking as soon as you notice the loss rather than waiting.
GPS carries privacy concerns as well. Location tracking can be abused for stalking, so app permissions for location need careful management. A workable rule of thumb is to review whether anything beyond apps where location is central, such as maps and navigation, has been granted "always allow." See also what to do first when a smartphone is lost and managing phone number privacy.