If you don't turn on the flight mode on the mobile while you are traveling in the plane, it will cause problems.
It doesn't mean the plane will crash if you don't put the phone in flight mode. But, it creates difficulties for the pilots who fly the planes. Having a mobile connection in the aircraft affects the communication system of the aircraft. During the flight the pilots are always in touch with the radar, control room. However, if the phone is on in this case, the pilots will not be able to get the instructions clearly. In such a situation if your mobile or laptop is switched on during the flight it will interfere with the radio frequency received by the pilot. That's why keep your phone on flight mode when you fly.
