Even on TRT, you produce some testosterone. (Among other things). It isn;t enough to maintain a healthy level but it's still doing something. When you do a cycle, that production is suppressed further. Normally, you want to get ALL of your levels back but obviously that doesn't happen on TRT, nor does it need to -- however, the actual PRODUCING of testosterone makes a difference in how you feel, the size of the testicles and several; other factors all associated with hormonal output -- all of which is disrupted when doing a cycle. By doing PCT, you get your body to normalize somewhat. If you just go back on TRT, you're not really replacing all the test in your system, you'll have LESS -- plus more estrogen , which affect LH and so on down the chain. Also, if you do HRT you can give yourself a little break and do a lower dosage for a few weeks until you ramp up back to your regular dosing.