Why drain water from the compressed air tanks?

5.1.4 – Air Tank Drains
Compressed air usually has some water and some compressor oil in it, which is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure that you drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom.
There are two types:
Manually operated by turning a quarter turn or by pulling a cable. You must drain the tanks yourself at the end of each day of driving.
Automatic--the water and oil are automatically expelled. These tanks may be equipped for manual draining as well. Automatic air tanks are available with electric heating devices. These help prevent freezing of the automatic drain in cold weather.
[CDL DRIVER MANUAL FINAL]