to prevent inside condensation. condensationis a matter of RH PSI and temp. you can get a puddle at 40F with a full tank llower the pressure and the dew point goes way down to sub -30s all the way to up to -100 or more. you can find a dew point calculator that includes psi. with out chemical filters on a compressor air in the tank is 99% RH at the temp filled wiwth chemical filters like desicant perhaps 50 or 60% RH at 3000 psi. if you vent the tank dow to say 12 bar the dew point can be as low as -100FWhy would you empty tanks?
Dew point under pressure | Airpack, custom made oil and gas packages world wide
go to this link and use the bottom part top entry -30 them 1 bar psi answer is -31 degrees for dew point
now use same -30 then at 200 bar and you now get a dew point at 41 degrees
raising the pressure changed dew point from minus temps to positive temps, in the winter in this case if the temps get to 38 degrees you will have water in your tank from condensation.