Why is Fatigue a Threat to Safety?

In short: because pilot fatigue can cause accidents and kill people on board and under the flight path.

Fatigue is a safety hazard because of the reduction in alertness and performance it creates. Fatigue is a normal response of the human body to many aspects of flight operations such as: sleep loss, shift work and long duty cycles. Its negative impact on flight crew performance can be significant as the crew have to remain alert to ensure a safe flight by their observations, communications and actions. The only effective treatment for fatigue is to get adequate sleep.