It need not be from an aeroplane. It could be from a yacht or boat in distress, or in this part of the world at least, a 4wd party in the middle of the 45Celcius desert who have broken down.
Aircraft monitor the frequency and report it when they hear something. The relative loudness of the ELT by different aircraft would help to locate the rough vicinity prior to the signal being picked up by a satellite and may expedite rescue.