It's for SAR as much as anything else.
Aircraft that is equipped with ADS-B avionics on the UAT datalink have a feature that allows it to broadcast an anonymous 24-bit ICAO address. In this mode, the UAT system creates a randomized address that does not match the actual ICAO address assigned to the aircraft. After January 1, 2020, and in the airspace identified in ß 91.225, the UAT anonymous 24-bit address feature may only be used when the operator has not filed a flight plan and is not requesting ATC services. In the anonymity mode, the aircraft's beacon code must set to 1200, and depending on the manufacturer's implementation, the aircraft's call sign might not be transmitted. Operators should be aware that in UAT anonymous mode they will not be eligible to receive ATC separation and flight following services, and will likely not benefit from enhanced ADS-B search and rescue capabilities.