The earth's atmosphere is composed almost completely of nitrogen (78%) and oxygen (21%), with trace amounts of several other gasses.
Water also occurs in the atmosphere (commonly in the vapor phase). The amount of water vapor varies greatly, depending on a multitude of factors.
The evaporation of water requires the addition of 80 calories of thermal energy per cubic centimeter. This heat energy is robbed from the local environment, and is stored in the vapor.
This heat energy remains locked up in the vapor until conditions change and the vapor condenses into the liquid phase, releasing the energy back into the environment.
This addition and loss of energy results in a global energy transfer which affects our lives, and the earth processes which make life possible (and pleasant). For example...
Perspiration: When our bodies sweat, the water which evaporates from our skin obtains the heat energy necessary for the phase change from the skin, cooling our bodies.
In comparison, dogs have no sweat glands, so they have no cooling mechanism. That's why they pant so much - all cooling comes from evaporation of water from their tongues.
Global energy transfer and temperature moderation: Far more solar energy (measured in langleys) is received on earth at the equator than at the poles.
Excess heat energy is used at the equator to evaporate seawater, which is then transferred north and south to mid-latitudes, where it condenses, rains, and gives up the heat to areas which need it. This tends to moderate surface temperatures on earth.
If you don't think this is important, try living on Venus or Mars, there temperatures can vary by several hundred degrees from sunlight to shadow, and day to night.
Adiabatic cooling and rain shadow deserts: As warm, moist marine air masses move onshore and rise up over mountains (like the Cascades) they are cooled at a rate of 5.5F°/1000' of vertical lift (called the "dry adiabatic rate").
This cooling continues until the temperature drops to the dew point when condensation begins. Above the dew point the temperature drop is reduced to 3.5F°/1000' (the "wet adiabatic rate") due to the additional heat energy released by the vapor as it condenses back into the liquid phase.
As the air mass (which is now dry) descends the far side of the mountain, it heats back up at the dry adiabatic rate, resulting in a warmer and drier air mass on the lee side of the mountain.