All living organisms require water and the water cycle describes the process of how water moves through the planet. Plants wouldn't grow without precipitation (and thus anything consuming the plants wouldn't survive and so forth). The water cycle brings water to everywhere on land, and is the reason that we have rain, snow, streams, and all other kinds of precipitation. Stopping it would cause an endless drought. No water flow in lakes would cause overgrowth, killing many species of fish and other lake wildlife.
