This is important in the atmosphere as we form ice, rain or water wapor. We release or consume energy which affects the temperature of the atmosphere. Weather, day to day variations in temperature, wind and precipitation. Climate, decadal averages over one region. Distinguish between relative and absolute humidity.
Water vapor can dissolve in air and can be transported around by convection.
Moisture can be redistributed over large areas. Water evaporates into air from seas, streams, lakes and soil. Air can only hold a certain amount of water vapor. That is it can hold an infinite amount of water. Warm air can hold more water than cold air.