Your body is about 70% water. Water in the body helps maintain fluid balance, transport nutrients, regulate body tempperature and digest food. The kidney filters about 200 quarts of fluid everday of which about 99% is circulated back and 1% is excreted by the body. Water is important for the functioning of the kidney, for dissolving the nutrients and ensuring that the bacteria, proteins etc do not accumulate. Therefore, ensuring that you get adequate water is essential and should become a habit.
For those who worry about clear skin, water is an excellent way to detoxify the body, especially warm water. Dehydration, which can occur if you lose even 2% of your bodies water content, can result in fatigue and muscle cramps. On a hot day or while working out, you are losing water faster than normal and therefore keeping that bottle of water handy is important. Not drinking enough water has also been shown to cause headaches. Obviously the most obvious benefit of water is that it dilutes or flushes out unwanted minerals from your body which can go on to become kidney stones.
The research on what is the optimal amount of water to drink throws up varied results. However if you are drinking less than at least 8-10 glasses of water, its probably not enough.