It's part of the flight plan Peter. Disregarding air pressure changes, if you plan a flight to (say) Chinchilla, you look at ERSA and see that Chincilla has an ELEVATION of about 1000 feet. So you plan to arrive in the circuit at 2000 feet indicated on your altimeter. However, especially on a longer flight, the pressure may drop (say) 10 millibars. That's 300 feet, and a drop in pressure means you will be flying that 300 feet LOWER in order to get the desired 2000 feet reading. You will actually be 700 feet AGL at the destination. Remember, from HIGH to LOW, lookout below!!!!