Final answer:
The use of an automated external defibrillator is critical for restoring normal heart rhythm in cases of ventricular fibrillation, which is an irregular and rapid heart rhythm that can lead to cardiac arrest.
Step-by-step explanation:
One reason for using an automated external defibrillator (AED) is to restore normal heart rhythm in a person experiencing fibrillation. Fibrillation refers to a rapid, irregular beating of the heart, specifically ventricular fibrillation which is a medical emergency. The AED delivers a significant electrical shock to the heart, intending to halt the arrhythmia, allowing the body's natural pacemaker, the SA node, to reestablish a normal sinus rhythm. Automated external defibrillators are user-friendly devices found in public places, with simple verbal instructions, enabling bystanders to provide essential assistance in the critical moments of a cardiac emergency.