The air around you contains moisture ( humidity ).
So compressed air has a moisture content which should be removed for driving sensitive and expensive equipment. The removed moisture collects in a container ( bowl ) and has to be removed periodically, either manually drained by inspection or timed intervals, or by an automatic system which can do it for you by sensing the water level in the bowl and opening the drain for release.
Other than that I am not sure what answer you are expecting.
Note that some diesel engines, if not all, also have a water collection system in the fuel line, Manual release is just turning a 'nut' at the bottom of the bowl to open the drain letting the water out. A sensor can tell you when the bowl is full. A glass bowl for old time gasoline engines let you look and see the level of the water level.
Those two systems are simple and ones I have been accustomed to.
So moisture collection is worthwhile and necessary for not just air compressed systems.