Why do the first 72 hours matter as nations ask citizens to be ready for emergency?

Several nations are urging citizens to prepare for emergencies by keeping essentials ready for 72 hours. The three-day window is intended to help households cope while authorities respond to a crisis. Power outages, severe weather and cyberattacks can disrupt services. During initial hours, responders may need to prioritise urgent tasks. Thus, households may have to rely on their own supplies.