Oceans soak up heat from the sun during the day, and release it slowly at night. So they help to cool the land when it is hot, and warm the land when it is cold. Ocean currents also warm and cool the land, while water from the ocean evaporates to form clouds which in turn, bring rain. The rain falls back into the sea. Even when rain falls on land, rain water is carried into the sea by rivers and streams. So, oceans are an important part of the water cycle too.
