It is because water needs the heat from the sun to evaporate into water vapour which then rises into the air and condenses to form clouds. Then the process precipitation(raining) occurs. So without the sun, water cycle won't occur.
The sun's energy is very important because it heats up the water to create evaporation
The water cycle is just the process of water rising into the air and raining back down again. The sun's energy helps the cycle by heating the water to allow it to evaporate into the air without the sun the only way water can evaporate is by sweating, boiling water, etc.
So the water can evaporate, from the sun's light, and the water may go up causing it to rain
The water cycle would end without the sun
So everyone can have day and night.
The sun is important because when the sun heat the ocean or other part of water it makes it to vapor.