Compressed Air Dryers

Why is a dryer needed for compressed air? 

All atmospheric air contains some water vapor, which will begin to condense into liquid water in the compressed air or gas system when the air or gas cools past the saturation point, i.e., the point where it can hold no more water vapor. The temperature at which this happens is known as the dew point. This dew point becomes all-important in determining how much compressed air drying is needed.