precious metal plugs were introduced for longevity - meaning you dont need to remove/replace spark plugs for at least 100,000 or 150,000 kms - The core reason was complexity of engine packaging e.g. FWD V6 engines where you need to remove the inlet manifold to remove the plugs from the rear bank - mitsubishi 6A series, toyota MZ, VZ, GR etc. Infact the mistu 6A12 and 6A13 engines with distributor use 3 platinum and 3 copper plugs - you can guess where the copper ones fit.
They do not increase any sort of HP or output, they are simply for longevity, I have used platinum plugs for more than 200,000 kms in different engines and they are still running fine. No difference in engine output, they start up every morning without complain.
The reason why Pakistani cars get copper plugs is that labor is cheap, you can get 3 or 4 plugs changed for less than a 1000 rs, try that somewhere outside of Pakistan.
Speaking of naming mods as stage 1, stage 2 etc. this started off with NFS and other games, and our current licensed generation do not know any better.