The HPTA, Hypothalamic-Pituitary-Testicular-Axis is the name given to the system of organs that regulate testosterone production in men. It's the thermostat for your body’s natural production of testosterone. Too much testosterone and the furnace will shut off. Not enough, and the heat is turned up, to put it very simply.
When you introduce supraphysical amounts of exogenous testosterone to your body, the HPTA shuts down/thermostat turns off. During PCT the goal is to "turn back on the furnace" and reboot our HPTA to begin producing our own testosterone once again.
However, TRT/HRT is not just for those individuals that are unable to reboot HPTA, but also for those that are simply low or not producing adequate amounts of testosterone and experiencing symptoms of Andropause.