Its a gimmick. Fuel to air ratios fall between the too rich and too lean for combustion, even then there has to be an ignition source in that critical % state in side the container.
It has never happened. Purging is just a gimmick used by suppliers to charge you for something you do not need. Big tanks like 500 gallon tanks are shipped and filled with air at atmospheric pressure. They are not vacummed or purged. The negligable air and oxygen becomes too rich of a mixture to burn,
Propane 101
Ask your local firefighter.
The potential water vapor could be a concern IF filled on 100% humidity day, IF used on much a colder day in which the tank empties to allow the water to boil. Otherwise, water will become and remain liquid until the pressure drops so low as to cause it to boil off or to freeze dry. That would be far below the boiling point of propane at -44.3 f Niether is ever likely or documented. It just cant happen
Talk to your local firefighter. not your propane supplier Its akin to asking to a used car salesman vs a scientist One will tell you its safe , the other truth. both will tell you its safe
Talk to your local fireman!