Fancy air compression systems (eg: not consumer) will have a drain valve that automatically opens when the pressure falls from turning system off for night/weekend/other maintenance so people don't forget to drain it.
Its also not just water that is collected at the bottom of the air tank, compressor oil also collects. Oil can also be deliberately added by either as a byproduct of compressor design or from separate equipment, into the airstream help lubricate and protect equipment from the moisture, reducing the need to remove the moisture through other means.
