Explain that why is it said like that?
Mock drill is useful.

Explanation

Mock drill is the practice to check whether there is preparedness for dealing with the sudden attack of disaster. For this purpose, virtual or apparent situations that simulate the disaster are created. The reaction time for any type of disaster is checked by such activity. In the presence of trained personnel, the execution of the rescue plans are observed. People also understand their responsibilities at the time of actual disaster. The experts also check execution of plan designed for disaster redressal. By such mock drills, the efficacy of the system can be understood. In future, when actual calamity strikes, there is already preparation for disaster redressal. Therefore, mock drill is useful.