• Drinking water helps maintain the balance of body fluids
  Water constitutes the majority of our body mass and sustains numerous vital functions. The brain signals to the kidneys, according to the content of water in the body, how much water to excrete and how much to store. When the ‘level’ of body fluids becomes low, thirst kicks in. That is the last reminder that water levels should be replenished. Alcohol interferes with the communication between the brain and the kidneys and is responsible for the excessive elimination of water, which leads to dehydration.
• Water assists in controlling the intake of calories
  Even though water does not mystically influence body weight, it does assist especially in the sense of substituting for calories which would otherwise be consumed through other beverages. Water in combination with foods which also contain lots of water helps to substantially restrict your daily intake of calories. Namely, the volume of foods with a high water content is usually larger (psychological effect) and the foods themselves usually require more chewing and are absorbed more slowly, which is why they make you feel full more quickly. Such foods include fruit, vegetables, soups and stews, porridge etc.
• Water energises the muscles
  Cells contract when not supplied with plenty of water. This leads to muscle fatigue and diminished physical abilities. The American College of Sports Medicine has published water consumption guidelines, which recommend that, even two hours before engaging in an activity, a litre of water should be drunk, and drinking water should be continued in regular intervals during as well as after the activity.
• Water will make your skin look healthier
  The skin contains plenty of water which acts as a barrier that prevents the excessive loss of fluids. When dehydrated, the skin is dry and wrinkled, though when hydrated sufficiently, your skin will be shiny and tight. What moisturisers do is to retain water in the skin, improving its appearance. Unfortunately, however, water will not make lines and wrinkles disappear. :)
• Water makes your digestion work smoothly
  Sufficient hydration helps in the proper functioning of your digestion. When not enough water is consumed, it is extracted from the faeces by the large intestine, which will lead to constipation. Sufficient quantities of water and fibre are the perfect combination – water transports the fibre through the intestine creating a sweeping effect.

Source: http://www.webmd.com/diet/features/6-reasons-to-drink-water?page=2