Using PoE (Power over Ethernet) to power cameras helps keep them running during a power outage because PoE cameras are centrally powered from the Ethernet switches, which are often backed up by an uninterrupted power supply (UPS). This means that in the event of a power failure, the UPS can provide temporary power to the switches and, consequently, to the connected PoE cameras, ensuring continuous operation.
Reference: Axis Communications - Power over Ethernet