Water moves in a continuous cycle between the land and the sky. The cycle starts from the ocean and the lakes when the temperature becomes hot so that the heat gradually evaporates the water into the atmosphere to form clouds. The condensed water vapor forms as tiny droplets into clouds, until one day it will be released as rain or snow from the clouds to the land.
Some of the precipitation falls in the oceans which continuous the cycle. However, some water is trapped under the land, this is called ground water. Some falls into the rivers or lakes, generally saying, the streams and rivers collect most of the water from the ground and travel back to the ocean. The water keeps its journey from the land to the sky, then the sky to the land.
This kind of cycle will never end so that the whole cycle can start all over again.
The benefit of it is that the slightly salty ocean water can always provide fresh water to the continents.