Remember early Microsoft Flight Simulator?
It had red boxes in the sky, you flew through them to achieve the result required ie approach to land.
High Key, Low Key etc are "Key" points (or "Gates").
Achieve each gate in sequence and you should achieve your objective ie PFL.
If you are not at the required height, speed and position, you may be able to continue your (now modified) pattern or have to abandon it - with all that implies.
It formalises a procedure and makes it more likely that you will be successful in your task, be it practice or real - it gives you an early warning too if things are not going "to plan".
lsh
