Why is it important to drain water from starting air bottles?

Water is present in the air. The amount will depend on the humidity of the air. When the air is compressed, and subsequently cooled in
 the air receiver, the water will condense on the cold surfaces and collect in the bottom of the receiver. If not drained off regularly
this water may cause corrosion in the receiver as well as being entrained in the air leaving the receiver and causing corrosion and
 subsequent jamming of main and pilot air valves.
During operation, air compressors lose some oil through piston rings. This oil along with moisture travels into the air bottle. Mixed with water, which condensed from moisture, oil forms an emulsion. A sticky emulsion travels into the control air circuits and makes control valves stick. This leads to starting and maneuvering problems of Main Engine and Auxiliary Engines as well.