Most of the additives change the cetane rating of the fuel. Which changes the burning characters of the Fuel.
US Diesel Fuel tends to ride on the lowest possible cetane rating which may or may not reflect what the manufacture intended. Then as in the case of my own 1984 vehicle the Diesel Fuel it was designed to run on is long gone and in an attempt to reduce emissions there has likely been 4 major changes in Diesel Fuel since that time.
People frequently say that when the do a diesel pure the engine quiets down (less of the diesel fuel knock) and after the purge their engine goes back to the normal sound when it is running on mostly only diesel fuel.
"Cetane number (cetane rating) is an indicator of the combustion speed of diesel fuel and compression needed for ignition. It is an inverse of the similar octane ..."
https://en.wikipedia.org/wiki/Cetane_number
The Fuel injection pumps that took the hardest hit from the changes in Diesel Fuel are those that have aluminum housing that have parts that slide against the aluminum. That would be Staynadyne/Roosamaster and the Bosch Fuel Injection Pumps that similar to the ones that go on the older VW Rabbits. Both types of Fuel Injection Pumps will so quite a bit of wear to the housings.
In your particular case it is kind of undetermined what the additive did to fix anything.
If after you stop using the additive it still is working fine it is likely you loosened something up. I not using the additive returns you to the problems you may need to retime the Fuel Injection Pump or do work in the Injectors.