But to be more specific:

* Protein supplements make it easier for your body to get the protein required for whatever demand you're putting on it. You need higher protein intake to growth than to sustain muscle. If you were on protein supplementation during a growth phase, and then transitioned to a maintain, and stopped supplementing, while making sure your actual diet was sufficient, then you could in principle maintain your muscle mass with no problem. The extrapolation of this is that in principle, you could always replace protein supplements with "real food", regardless of phase and get pretty much the same benefits. There may be lifestyle/ease/cost tradeoffs.

* Creatine supplementation helps your body get "a little extra" work done. The direct boost that creatine grants you is real, but also like... small. I think it's often visualized as "it helps you get that one extra rep/half rep". Where creatine pays off is that by sustainably helping you get that one extra rep, it helps create greater demand on your body over time, therefore promoting more growth. If you stopped creatine supplementation, you would probably lose that "extra rep", but all the extra growth that you got would be locked in.

* Caffeine... well, that's far more psychological. Leaving the world of sports, as a knowledge worker, I've committed myself to my addiction. The withdrawal is real, and even if I worked through the headache, I definitely do not feel as sharp.