As you know, takeoff and landing are the most difficult stages of the flight, during which pilots need to coordinate their actions with the flight control center on the ground. All this is done using the aircraft’s navigation system. The signal of your gadget can cause interference in the radio communication, and the pilot will not hear information transmitted by the traffic controller. This might lead to an emergency situation.
In flight mode, your gadget disables all data services (Wi-Fi, GSM, Bluetooth, etc). Simply put, your phone or tablet ceases to function as a radio receiver. If your smartphone has not been switched to flight mode, then the signals emanating from it may interfere with the highly sensitive electronic devices of the aircraft.
Furthermore, When a smartphone is locked and you’re not using it, it is still searching for a network, and its frequency may overlap with the frequency of aircraft signal. Even if you think a "sleeping" gadget does not pose a threat, switch it to flight mode. If your device doesn’t have this functionality, just turn it off.