A drain valve performs the critical function of allowing excess water vapour to escape into the atmosphere. The removal of condensation helps prevent the formation of corrosion that will eventually damage the compressor and shorten its lifespan.
Some compressed air auto drains work on the float principal. As water and contamination accumulates in the bottom of the bowl, it will ultimately lift a float. The compressed air, under pressure in the bowl, will then vent to atmosphere through the drain opening, blowing the water and debris from the bowl as it does.