Why purge...
New propane tanks for sale like a 20 lb (5 Gallon) are generally shipped with air pressure, sometimes as much as 100 PSI, to provide a balance of pressure while being shipped over various altitudes.
Air may contain certain levels of moisture content and other contaminants.
Water vapor present in propane gas vapor may cause regulator freeze ups and other problems.
Failure to purge the high air pressure will prevent the tank from being filled completely.
Air left in a propane bottle will negatively affect the burners on your appliances causing the flame to be unstable.