Fail to comply with the instruction to turn your phone to flight mode and your aircraft will not plunge from the sky, divert to Broome instead of Brisbane or otherwise do something untowards, but the reason is that phone signals can interfere with communications between the cockpit and the control tower.
When a mobile phone is searching for a connection, it makes a buzzing noise that can be picked up by the pilot's headphones.
That's especially the case during take-off and landing, which are the most critical phases of a flight.
On every flight, there is a strong likelihood that not all passengers have switched their mobile phone to flight mode.
If there are 25 passengers who have their phones activated or who might be texting, that can be a lot of static.
While the chance of these noises causing an instruction from the control tower to be misinterpreted by a pilot is extremely low, it's annoying for the person flying the aircraft, and a calm pilot who is not distracted by annoying noises is what's wanted.