What Will Happen If You Don't Put Your Phone On Airplane Mode While Flying?
Airplane Mode disables all communications protocols on your phone which leads to a Cellular communication along with WiFi, GPS, and Bluetooth.
It is an icon in notification bar.
even though there is an airplane mode, some flights do offer Wifi even when this mode is switched on.
It saves your battery from draining.
Airplane mode can also help you sustain battery for longer duration. Thus, you can switch on the mode even when you are on the ground, whenever needed.
This sound must be annoying to Pilot, which is why they ask to switch off the phone.
Some people might forget this, although! Which they should not.