When your water intake does not equal your output, you can become dehydrated
Water—whether by itself or from foods or other fluids—is essential to your health. "Think of water as a nutrient your body needs that is present in liquids and foods. All of these are essential daily to replace the large amounts of water lost each day," says Joan Koelemay, RD, dietitian for the Beverage Institute, an industry group.
Hydration is important whether you’re exercising, traveling or just sitting in the sun.
Fluid losses occur from evaporation through the skin, breathing, urine and stool, and must be replaced daily for good health. These losses are accentuated in warmer climates, during strenuous exercise, in high altitudes and in older adults, whose sense of thirst may not be as sharp.
The benefits of water throughout your body
Your body is composed of about 60% water, which helps maintain the balance of body fluids. Digestion, absorption, circulation, creation of saliva, transportation of nutrients, and maintenance of body temperature are all reliant on adequate hydration.
Keeping the body hydrated helps the heart more easily pump blood through the blood vessels to the muscles. Water helps energize muscles to maintain their balance of fluids and electrolytes. It improves the quality and appearance of the skin, which functions as a protective barrier to prevent excess fluid loss.
The kidneys do an amazing job of cleansing and ridding your body of toxins as long as your intake of fluids is adequate. If you chronically drink too little though, you may be at higher risk for kidney stones, especially in warm climates.
For most people, drinking water is the best way to stay hydrated. Whatever drink you choose, be mindful of its sugar content, and avoid beverages containing alcohol, which can cause dehydration.
Source: http://www.webmd.com/diet/features/6-reasons-to-drink-water#3